I would like to say before everything, I'm really enjoying the discussion! :)
First of all, I think we need to start with addressing the terminology being used and the exact breakdown of "Snape's Worst Memory" What James and Sirius did to Snape—ambushing him, using Scourgify to make him choke on soap, hanging him upside down, and threatening to strip his pants off— exposing his underwear to the student was a horrific display of bullying and public humiliation. It is indefensible and gives u all the right to hate James for it. However, labeling a teenager's threat of public stripping as "sexual assault" isn't the appropriate vocab. Sexual assault in its legal definition implies a motive of sexual gratification or violation, whereas James's motive was purely arrogant intimidation and public humiliation.
Furthermore, it’s worth noting that the spell used to hang him upside down, Levicorpus, was actually invented by Snape himself to use it against his enemies— the marauders. As Lupin stated in The Half-Blood Prince, it was a spell that became a popular trend where everyone was using it on each other.
Also, the spells James used weren't "illegal"—they were common school hexes. (Snape himself saw Draco using some of them against other students, and didn't say anything about it.)
Death Eaters Were Not Just a 'Cult' !
You are labeling teenage bullying with extreme terms while simultaneously watering down Snape's willing participation in a literal terrorist organization as just 'joining a cult.'
They were a genocidal terrorist organization. Joining them meant actively participating in or supporting the torture and murder of innocent people solely based on their blood status.
Snape didn't leave because he suddenly realized their ideology was wrong. he did that just because Lily was personally targeted. In the DH, we saw him deliver the prophecy to Voldemort fully knowing it would result in the murder of a baby and its family.
Take a sec to think about it, what could be worst than being the reason of murdering an infant! that was his choice.
What James did is incomparable to this.
If Voldemort said that the prophecy is pointing to Neville Longbottom, Snape would have remained a Death Eater and let the family and the infant die without a second thought.
That's bring us to the next point, the argument that Snape was simply brainwashed by Slytherins from age eleven ignores his own agency and his pre-Hogwarts behavior. In the chapter "The Prince's Tale" before they even step foot in Hogwarts, Lily asks Snape if it makes a difference that she is Muggle-born, and Snape hesitates before answering.
Talking about the environment and 11 years old Snape didn't have a choice, how do we explain Sirius Black or Andromeda Tonks? They were raised in the most extreme, abusive, pure-blood supremacist families imaginable from birth, yet they rejected those ideologies entirely. Equating the Marauders' bullying to the actions of Snape's friends is a false equivalence. James was a bully, but Mulciber and Avery were literally practicing terrorism and dark magic.
You also questioned how Snape could possibly know dark curses at age eleven if he grew up in a Muggle neighborhood with an abusive, magic-hating Muggle father. The answer is directly in the text, his mother, Eileen Prince, was a pure-blood witch. Snape deeply resented his Muggle father and heavily identified with his mother's magical heritage. It is entirely logical, and supported by canon, that he consumed whatever magical books or knowledge his mother possessed before going to school.
Even if we assumed that Sirius is just an "unreliable source." We don't even need his quote to prove this, Severus was a prodigy. He was successfully inventing and casting non-verbal spells—highly advanced magic typically taught in the sixth year—when he was only 14 or 15. He was also inventing incredibly dark curses. This canonically proves he had an exceptionally advanced grasp of the Dark Arts and spellcraft from a very young age, completely validating the claim about his early knowledge.
You also question how James's character development can be proven by his appointment as Head Boy, pointing out that Dumbledore made Draco Malfoy and Remus Lupin prefects despite their flaws. However, there is a massive difference between a fifth-year Prefect badge and the seventh-year Head Boy position. Prefect badges are often given strategically, Remus was given one in a failed attempt to control his friends, and Draco was likely recommended by his Head of House, Snape. Head Boy, on the other hand, is the highest student authority representing the entire school. Dumbledore, who knows everything happening in his castle, would never give that ultimate leadership role to a seventh-year who was still actively and publicly tormenting people for fun.
Regarding the Boggart, I see your point but if we have to ignore the JKR's world-building to defend a character, we are no longer discussing Harry Potter. Lupin explicitly states in PoA that a Boggart takes the shape of "whatever frightens us most." most profound fear. Lupin's greatest fear is the full moon because it represents his transformation into a monster that could kill his friends. Hermione's greatest fear is academic failure, which McGonagall represented in that moment, telling her, "You've failed everything!". Neville’s greatest fear wasn't Bellatrix Lestrange—the woman who tortured his parents to insanity—it was his teacher, Severus Snape.
Neville didn't fear Snape as an abstract symbol of 'stern authority'. he feared him because Snape was a direct source of psychological terror in his life. We cannot just erases the very real, targeted abuse Neville suffered at his hands. (He wasn't even able to read instructions out of fear! Yet he passed the owls potion exam when he wasn't there.)
Snape did not take a "calculated risk" with Trevor the toad, he maliciously told Neville he would feed the botched potion to his pet at the end of class, knowing Neville lacked the skill to fix it. He then threatened Hermione not to help him. When the potion worked and the toad survived, Snape was not relieved, he was angry and deducted points from Gryffindor because Hermione had helped. He fully intended to poison a student's pet. However, a grown man in his 30s mocking a 14-year-old girl's physical appearance to the point of tears isn't 'bad at jokes' or 'insensitive.' It's deliberate emotional cruelty from a teacher in a position of power.
When you said Also that Lily didn't care that James's friends almost killed Snape is factually incorrect because she didn't know the truth. Dumbledore swore all parties involved to strict secrecy regarding Remus's condition and the prank. Lily only knew that James saved Snape's life and Severus wasn't able to tell her all what happened because of this.
Finally, claiming that Snape 'didn't have a choice' completely erases his agency and the entire point of his redemption arc.
No one could deny Snape's bravery, nor could excuse the Marauders' horrific bullying. But loving a grey character means accepting the weight of their choices. Snape wasn't a passive victim of circumstance, he actively chose the Dark Arts and joining death eaters, he chose to deliver the prophecy, and later, he heroically chose to sacrifice his life to defeat Voldemort. Stripping him of his free will just to paint him as a helpless victim doesn't defend him—it ruins the very complexity that makes him such a brilliantly written character.
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