THIS.
Just want to remind everyone that Snape was abused as a child. He found a friend in Lily because she was like him. A magician. Then at school, he faces the same abuse by James and his asinine gang. So of course when shining prefect and star pupil Tom Riddle shows Snape kindness, Snape begins to adopt his view points. Tom was a sociopath, a person incapable of empathy or love. He could easily manipulate people with low self esteem. And to add into the mix, the one other person who was ever nice to Snape, ends up dating his abuser. Snape was not a “nice guy.” He was a poor abused child who fell under the sway of a charismatic nut job.
Just want to remind everyone that while Snape may have been abused as a child, so too were Sirius by his family, and Remus by his peers. Snape was abused and found a friend in Liiy, but he also hurt her sister before Hogwarts, and while being defended from the Marauders by Lily referred to her as a derogatory slur because she had drifted apart after years of mingling and mixing with people other than Snape. Even if Snape was a guiltless abused little boy, that doesn’t excuse the behavior he exhibits as a teacher seeking redemption for past deeds. It doesn’t help matters that all the scenes regarding Snape’s past are told from his memories with his bias and still don’t excuse his actions.