SNAPE JOINED THE DE IN HOPES OF IMPRESSING LILY
I was thinking about this point since I'm working on the new chapters of TSOTP in which I try to include as many canonical things as possible in a coherent manner.
As you know by now, I consider direct statements from the author to be canon, so things like: "He (Snape) wanted Lily and he wanted Mulciber too." or the fact that James was jealous of Lily and Severus's relationship are canon
The point is that inserting this into the story in a credible way isn't easy. Let's start with the obvious: Snape is 16 years old, he's not a child, but he's not an adult either. I remember being 16 quite well, and looking back, I realize how immature I was, at least compared to today. Furthermore, in the story, in 1976/1877, the first wizarding war was about to break out; they were already in a state of war, even if it wasn't explosive. Voldemort was rising to power, albeit in the shadows.
The Death Eaters were already known and active, as were their ideals, yet the fact that they were evil is just a vision.
Calm down, calm down, I'm not really justifying the Death Eaters; they were horrible people, disgusting classists who wanted to hunt Muggles for sport. But in the narrative fiction, they thought they were right; every good villain thinks they are misunderstood heros, and that's what makes the story fascinating.
For this reason, reading and writing about indoctrinated children shows us how sadly understandable their siding with the wrong side actually is. Even today, extremism is active in the real world, and by trivializing the difference between right and left, no one truly believes they're wrong; everyone, or almost everyone, is convinced that anyone who opposes their political faction is the wrong extremist. So, in their minds, future DEs are heroes against the grain.
The fact that most people are against their ideals makes them fall, in a distorted way, into the classic "the difficult choice is the right one, conforming to the crowd is the lazy and wrong choice." When Dumbledore says: "We must choose between what is right and what is easy" future DEs interpret it as: "You see, we are frowned upon because we subvert the system. It would be easy to do as the others do, but we choose the difficult path, and therefore we are right."
And this is where Snape's tragedy solidifies.
He isn't just a boy who needs belonging and protection; he, like everyone else, wants to be useful, to prove that he matters and that he has courage, the courage to fight for what he believes is right. From this perspective, the author's statement makes perfect sense. He doesn't think he'll impress Lily by torturing people like her, Muggle-borns, because he doesn't yet know that DEs do that, canonically. He thinks that DEs fight to defend wizarding rights. And that opposing the superficial do-goodism will truly bring protection to wizards. For Severus, Muggles are dangerous; just look at how his father treats him and his mother. For Severus, growing up with Muggles is a problem that makes him ghettoized; an all-magical society would be preferable. Of course, it's unfeasible, and above all, it wouldn't lead to the results he hopes for, but for a sixteen-year-old, this idea makes sense, especially when most of his classmates who protect him have those same ideals. And so when Muldoon (the sixth-year Defense Professor I created for my FF) makes this speech:
"There's a huge need to be accepted and feel part of something. We're all social beings. But this fear of being perceived as different pushes us to conform, and there's no worse way to kill each other's uniqueness."
He saw Lily nod strangely; he had the distinct impression that she didn't like Muldoon at all. “Remember, different doesn't mean wrong, not even in the dictionary. Each of you has your own pace, your own rhythm, and your own mind, and you must have the strength to think with it. I know it's much more comfortable to trust someone else to make decisions for you. Following the crowd is easy, but very often the easy path isn't the right one.”
That simple sentence triggered something in his mind; it was so obvious, yet so enlightening.
Everyone seemed to have the truth in their pocket, everyone knew what was right, but in reality it was all appearance, ostentatious do-goodism. All the talk of Squibs' rights, defending Muggles, it was all just a facade, a way to avoid feeling out of place, different.
Mulciber had told him, many wouldn't understand what his father was doing, what The Lord was fighting for.
Most were against it, but those who had the strength to think for themselves agreed with him. Wizards had to be defended, the wizarding world had to be protected.
The more difficult path was the right one.
He briefly met Lily's gaze; she too seemed convinced, moved by Muldoon's words. She probably couldn't truly join the cause, she was too indoctrinated, too caught up in that rampant do-goodism, but deep down she knew it was the right path.
Perhaps, he dared to think, if he truly became an active soldier in defense of wizards, she would be impressed.
From chapter 33 The Story of the Prince
Snape interprets it completely differently from Lily, but seeing that she also seems convinced and nods, he makes the fatal mistake of thinking she'll be impressed if she sees him fighting in a war she can't actively participate in. And that's the real tragedy.
But before writing the chapter, I did some research, and I'm still doing it, and I've come across some very interesting counterarguments, for example:
It’s poor writing, because Rowling doesn’t show in the books what would make Snape think that Lily would be impressed by him joining a hate group AFTER she told him off for wanting to join a hate group.
This actually makes sense from an outside perspective; we know how the story went and how it ended, just as we know almost everyone's points of view. So, of course, fifth-year Lily ends her friendship with Severus, partly because of his closeness to the future Death Eaters, and she's already guessed he wants to join them, but she doesn't know why. She thinks she knows, but she doesn't wait for his response.
So, once again, tragedy strikes: they have seemingly opposing views on the same issue. Lily sees the DEs as a hate group, Severus sees them as defenders.
Is Snape stup*d, then?
No, it's simply the opposite of the point of view. Try to PUT YOURSELF INTO THEIR PERSPECTIVE.
Let's leave aside the DEs and the adults and just look at the student groups. For Lily and the others, the Marauders are arrogant kids (James and Sirius more than anything). Okay, in some cases they went too far, but practically everyone judged them. Lily herself draws the line at not using dark magic, so for her, the illegal spell used on Bertram isn't that serious, the SA she suffered from Snape isn't that serious, while the attacks the Slytherin gang uses are extremely serious. What Mulchber TRIED to do to Mary is much more serious than what the Marauders actively did. She's not silly, she simply has this view, she has a different value system and experiences, so for her, the Slytherin gang is worse than the Marauders.
For Snape, who has actively endured a lot of bullying and aggression, Marauders are much worse than some of the insults his friends use. Neither Lily nor Severus is stup*d or evil; they're simply on different sides, and neither is making the effort to truly understand the other. And they both hope the other will change his mind: Lily hopes the ultimatum will push Snape away from future DEs and the dark arts, while he hopes to impress her by showing her that he's brave.
And as silly as this may seem, I repeat, to a 16-year-old the connection is clear. James Potter, whom Lily considers arrogant and annoying, earned points in her eyes when he heroically saved Snape from a threat she doesn't even know about. The worst bully in school, who curses his classmates out of boredom, had impressed Lily long ago, so why shouldn't Snape actively fighting in a war impress her just as much, if not more?
I repeat, it's obvious that it's absurd in our eyes, because we know the whole story and have a clear vision.
I'm going to risk making a point that's factual but absurd, but be careful, it's not meant to be an attack, it's just to explain.
Let's say you're an omnivore and you eat meat. That's fine with you; there's nothing strange about it, you've always done it. However, a movement for vegan rights is growing, wanting to defend their right not to eat meat and have other foods.
In doing so, however, they organize demonstrations that block access to meat shops, thus prohibiting you from buying your food.
They use force to limit your rights, and you get irritated. They accuse you of being a mur***er because you eat meat.
A reactionary movement for the rights of meat eaters is forming, and you join in. You don't want to prevent vegans from eating what they want, you don't want to force them to eat meat, you just want you and people like you to be free to eat meat.
Your joining that movement doesn't make you a monster, just as those who fight for vegan rights don't. But then everything degenerates, the tone becomes harsher, and what were intended to be defensive movements become offensive, personal inclinations become a reason for violent struggle, and your desire to eat chicken makes you the equal of Hannibal Lecter. Conversely, those who fight for the right to eat tofu in peace and defend animals are made out to be crazy people who want to force everyone to be vegan and who would have all meat-eaters arrested.
Obviously all this is absurd and trivialized, but I hope it helps us understand how the vast majority of people who participate in reactionary movements aren't extremist in nature but are lumped together. They don't join together to attack but to defend.
When things get too far ahead, even the memory of who started them and why is lost. What remains is the war.
And if you join halfway (as happens in the Marauders' area) you don't have a clear idea; you don't know who escalated the conflict.
We know it was Voldemort in the shadows, who's been working on this since the 1940s with the first DEs, but the general population doesn't know this; they only see the result.
So ultimately, I think the author's statement makes perfect sense for young Snape, and in fact makes him even more nuanced than many fans see him. It saddens me to see how many trivialize him by calling him stupid or senseless or poor writing, when in reality it shows his humanity.