Snape joined the dark side because âLily rejected him.â Snape joined DE because he wanted to kill Muggleborns.
First, a fair few, in the first war, didnât even know what Voldemort really did.
They werenât alone either, there were quite a few people, before Voldemort showed his true colours, who thought he had the right idea about things⌠They got cold feet when they saw what he was prepared to do to get power, though.
It was not his ideology that people found distasteful, it was his methods.
There were many reasons people followed Voldemort, and most were not because they âwanted to kill Mugglebornsâ.
But letâs look at things from a Snape perspective for a little.
To Snape, In his eyes, in his point of view - the light side was the dark side.Â
What the f did the light side ever do to help him? I donât see Dumbledore helping Snape and get him to stay on the light side. Dumbledore protected his Gryffindors and said f you to anyone else.
On one hand, Snape had the âlightâ side, who rejected him, abused him, made his life hell, tried to kill him and the only punishment from that was to Snape to keep his mouth shut about it or be expelled. And then there was Snapeâs Worst Memory which was probably the final nail in the coffin.
And on the other hand, there was the 'darkâ side who accepted him, were his 'friendsâ, respected him, made him feel wanted, gave him something he always craved acceptance. I know what side I would choose (p.s it wonât be the light side). Snape didnât just wake up one day and say, âI know, Iâm going to be a Death Eater.â The dark side saw a vulnerable young boy, groomed him and then sucked him in.
Not every single DE was one because they believed in Voldemortâs Pureblood stuff or joined him out of choice. Some joined out of fear for their life, or families. Some were vulnerable and manipulated, some had no choice.Â
 âThis has been a terrible ordeal for her. There will be no punishment. Older and wiser wizards than she have been hoodwinked by Lord Voldemort.â
Anyway, this â this wizard, about twenty years ago now, started lookinâ fer followers. Got âem, too â some were afraid, some just wanted a bit oâ his power, âcause he was gettinâ himself power, all right
âVoldemort doesnât march up to peopleâs houses and bang on their front doors, Harry. âHe tricks, jinxes and blackmails them. Heâs well-practised at operating in secret.â
âAs he moved up the school, he gathered about him a group of dedicated friends; [âŚ] They were a motley collection; a mixture of the weak seeking protection, the ambitious seeking some shared glory, and the thuggish gravitating toward a leader who could show them more refined forms of cruelty.
In my view, Snape (and Peter) was a combination of the first two, (weak seeking protection, the ambitious seeking some shared glory,)
Snape (and Peter) showed no signs of being a thug at school (being interested in Dark Magic does not make you a thug, defending yourself does not make you a thug. Laughing at a curse your friends did does not make you a thug, (or we can put Lupin in the category of thug, and many other characters, including H, R & H).
And then Lily was put in the crossfire, at which point he forgoes protection and glory and goes over to the losing side of the war, the side where:
âLast time we were outnumbered twenty to one by the Death Eaters and they were picking us off one by one âŚ
He spied on the darkest wizard, putting his life at risk every day against a man who.
â⌠shows just as little mercy to his followers as his enemies.âÂ
The Dark Lord, for instance, almost always knows when somebody is lying to him.
He spied on a man who was:
[âthe Dark Lord, the greatest wizard, the most accomplished Legilimens the world has ever seen?â]
You donât just hand in your resignation to Voldemort. Itâs a lifetime of service or death.)
Giants, werewolves and Dementors didnât side with Voldemort because they wanted Muggleborns dead.
 "How come they (werewolves) like Voldemort?â
"They think that, under his rule, they will have a better life.'Â
Envoys to the giants?â Fudge shrieked, finding his tongue again. âWhat madness is this?âÂ
âExtend them the hand of friendship, now, before it is too late,â said Dumbledore, âor Voldemort will persuade them, as he did before, that he alone among wizards will give them their rights and their freedom!â
 [âI mean we soon found out he didnâ object ter all wizards â just us.â
âDeath Eaters?â said Harry quickly.
âYep,â said Hagrid darkly. âCouple of âem were visitinâ him evâry day, bringinâ gifts ter the Gurg, anâ he wasnâ dangling them upside down.â]
The light side treated werewolves, and giants, worse than shit! So what do they do? Go to a side where they believe theyâll be better treated, the side who wasnât using magic against them. The side who promised them rights and freedom.
âThey (dementors) will not remain loyal to you, Fudge! Voldemort can offer them much more scope for their powers and their pleasures than you can! With the Dementors behind him, and his old supporters returned to him, you will be hard pressed to stop him regaining the sort of power he had thirteen years ago!â
There are hints that Lily might have joined Voldemort.
Not Harry. Please, not Harry. Iâll do anything!
Well, We all know what anything can mean. And Voldemort wanted Lily to join his side, but she turned him down.
Voldemort wanted powerful people, he wanted numbers: the more numbers the stronger the army - He didnât care if you were human, he just wanted numbers - he wanted Lily, a Muggleborn.Â
In response to an interview request for details about how the Potters âthrice defiedâ Voldemort, she claimed that he recruited them and they turned him down:
JKR: It depends how you take defying, doesnât it. I mean, if youâre counting, which I do, anytime you arrested one of his henchmen, anytime you escaped him, anytime you thwarted him, thatâs what heâs looking for. And both couples qualified because they were both fighting. Also, James and Lily turned him down, that was established in "Philosopherâs Stoneâ. He wanted them, and they wouldnât come over, so thatâs one strike against them before they were even out of their teens
Below, Slughorn implies he would have joined Voldemorts side, out of fear.
â[âŚ] Are you really telling me that they havenât come recruiting yet?â
âI havenât given them the chance. Iâve been on the move for a year. Never stay in one place more than a week.
Above, tells us that Dumbledore, Hagrid, and Sirius all recognise that not all Death Eaters were racist thugs who were interested in flexing their muscles in more refined forms and killing all Muggleborns.
Above shows that if you treat someone like shit then donât expect them to come at your beck and call when you need them. Itâs the saying of too little too late, you reap what you sow.Â
When Sirius tried to kill Snape, Snape was the one who was punished. Unfortunately, awarding Sirius a second chance robbed Severus Snape of his first chance.
The Prank is doubtless one of the main factors that drew Snape to the Dark side. After his falling out with Lily, he had no one to turn to except his fellow Slytherins.
Snape hung around with DE offspring Slytherins, just like Lupin hung around with James, Sirius and Peter, and both did it for exactly the same reasons; they accepted them. They had to share a dorm with them for 7 yrs. The last thing Snape needed on top of James and Sirius bullying him was his Slytherin dorm mate hating him too.
Look how much Lupin let James and Sirius get away with? Look how he (a prefect) didnât stop them from doing what they did to Snape,? He was clearly uncomfortable with what they did, but he didnât stop them. Why?
Because Lupin knew what it was like to be so lonely, to have no one, he didnât want to go back to that. He was scared he would lose them as friends if he did step in, and I donât blame him, just as I donât blame Snape for hanging around who he hung around with and laughing at their âpranksâ. And he really couldnât afford to not be friends with JP and SB.
After all, Lupin knew first-hand what the wands of JP and SB can do. And, he feared that in the flick of a wand, they could turn his world upside down if he upset them, let out his secret.
It is exactly the same as Snape and his dorm mates. He had no one but his dorm mates. And I would not want to share a dorm with the offspring of DE or DE wannabes (or anyone) for 7 yrs and have them hate me.
Look how it was for Harry in the few months he and Ron fell out and him and Seamus? When Harry fell out with Ron? Harry had it rough. Hated every second of it.