(this got away from me, but as a previous Snape Stan I see it as my right to roast him)
Regulus grew up in the house of black.
Regulus dealt with the pressure of being the perfect son. He dealt with a family full of blood supremacists, who had always told him muggleborns were bad.
Regulus was a boy when he joined and a boy when he died. He took initiative once he realized what was happening. Regulus may have figured it out because of Kreacher (and been upset about it) but he went into that cave because he knew there was something much darker than curses hidden inside it.
What took Dumbledore years, took Regulus months. He was smart, and maybe a bit cowardly. But he went into that cave expecting to die with no recognition. He gave his life in hopes to get rid of the horcrux. Did it make things more difficult in the end? Yes. But he still tried. He died trying to end Voldemort.
In addition: Regulus was shown being kind to Kreacher (and presumedly all house elves). Snape was shown being kind to one muggleborn, and then he joined a group onown for attacking them.
Snape was a half-blood. His father was a muggle, which may explain his hate towards others like him. However this doesn't excuse his actions.
He was obsessed with Lily. You hear me? Obsessed. That wasnt love. Ripping a photograph so that a woman's laugh is for you isn't something normal people do. He may have had a hard life but it doesn't excuse his actions.
He threaten to poison a kids pet. He bullied Hermione until she changed her teeth permanently. He was unjust to any house but Slytherin. He is everything bad a Slytherin can be in one person.
If Neville had been the one Voldemort tried to kill, Snape would still be a deatheater. He said for Voldemort not to kill Lily, as if she could be happy without her husband and son. Stop excusing his actions because he was "in love" with Lily.
James Potter, Sirius Black, Remus Lupin. They were all dumb, immature, reckless kids. But they changed. Snape stayed immature. He stayed bullying others. Snape wasn't good.
Regulus turned because he saw what Voldemort was doing, and decided it was wrong.
Snape turned because Voldemort killed the girl who was his "exception".
