Let's talk about Snape. There are many opinions about him, mostly because he is a very complex and interesting character. I want to say my opinion about him.
First of all, he had a terrible childhood. At home, with his parents, and at school. He was a very rejected kid and was bullied by James and Sirius and many others, if I remember correctly. He was humiliated several times and he could've been killed once, when James and Sirius made foam come out of his mouth that made him choke. A very traumatic childhood, I think we can all agree about that. Later, he joined the death eaters because he wanted to. He has always loved the dark arts.
When Lily was killed by Voldemort, Snape left the death eaters, because he was angry and sad that Voldemort killed the woman he loved, or rather was obsessed with, and because Voldemort was "dead". Then Dumbledore gave him a job at hogwarts as a potions teacher. He decided to bully his students (literal children) because he was bullied when he was a kid. He was even Neville's bogart - the thing he feared the most in the world. Not Voldemort, a murderer, not Bellatrix, the woman who tortured his parents. His potions teacher - and that says a lot. Something that stuck with me from my last re-read was when the trio was fighting a bunch of kids from slytherin (I believe in the goblet of fire but I'm not sure) and some kid cursed Hermione and made her front teeth grow downwards. Now, Hermione was already insecure about her front teeth, and when Snape came to see what happened he looked at Hermione and said that he doesn't see any difference. That is one of the meanest things you can do to someone - laugh about their insecurities. Not to mention that he's a grown man. A teacher. And she's a 14 year old. That's just messed up. And there were a lot of other cases when Snape bullied his students.
Now, let's move to his "redemption arc". We all know that Snape was a double agent. He worked for Dumbledore and pretended to be loyal to Voldemort. But he didn't do any of the good things he did from a place of generosity or caring. The way I see it, he had two motives. His first motive was to make sure that Lily didn't die for nothing. All of the times he saved Harry weren't because he liked him, or cared about him. This motive was relevant from the moment Dumbledore met him after Lily died, until the moment Dumbledore told him that Harry is going to have to sacrifice himself. Then, this motive was no longer relevant. Harry would die anyway, whether Snape is going to keep being loyal to Dumbledore after he dies, or whether he isn't. Either way, he's not going back to Voldemort because he killed Lily and he will forever be mad about it, so why did he still do what Dumbledore asked him to do? Because he was loyal to Dumbledore. And that's his second motive. He was loyal to Dumbledore because Dumbledore did everything for him. He gave him a job at hogwarts, he stayed loyal to him even when people thought that he was still a death eater, he was kind to him. And Snape returns the favor to the people who do good things for him, and we can see that with Lupin too: when the death eaters ambushed the trio and the order of the phoenix in the deathly hallows, this death eater tried to hurt Lupin and Snape cursed that death eater and saved Lupin, because Lupin was nice to him and apologized for what he did, or didn't do when they were kids. So, Snape clearly is loyal, and that is a good thing I see about his character.
So, I think Snape has changed a little bit. His intentions were never about saving Harry or the Wizarding world. He didn't care about that. If Voldemort didn't kill Lily, Snape would still be a death eater. Snape's motives were selfish, and never for the greater good. One time it was for the woman he loved and one time it was for the man he was loyal to and appreciated (and also loved). But I do think he has some good in him. not much though, but some. I think that he doesn't deserve people to praise him or even like him because if he did good things, but the intention behind them was wrong, it doesn't count. I'm not saying that if you like Snape you're wrong - everyone has their opinion and that's okay. ♡