Throughout centuries the House of Slytherin has attracted ambitious and cunning students seeking for success and personal glory. It is the house favoured by old pureblood families, such as the Malfoys or the Blacks, and the house the has apparently produced more dark wizards than any other. In the following article I will give the reasons why I think Lily Evans should've been a Slytherin.
Lily is a Gryffindor, and a fitting one. Harry Potter's famous mother and reason why he was able to put an end to Voldemort, was a talented muggleborn witch who attended Hogwarts in the 70s. She was teacher's favourite and a member of prof Slughorn's elite Slug club. After finishing Hogwarts she joined Dumbledore's Order of the Phoenix to fight Voldemort.
Lily Evans was a friend of none other than Harry's most hated prof Severus Snape, a halfblood Slytherin. The lived in the same neighbourhood before they went to Hogwarts and Snape was the one who introduced her to magic. Having a witch for a mother, Snape knew something about the magical world, inculding about Hogwarts. On the other hand, Lily was the only one aware of his tragic situation at home. The two were very close in their early childhood, but parted at Hogwarts when they were sorted in different houses.
Severus Snape probably wanted to be sorted in Slytherin. Most purebloods do and while he wasn't one, his mum comes from an old wizading family. Lily, on the other hand, knew nothing about Hogwarts. The things she knew were the things Snape told her. If he wanted to be sorted into Slytherin, he certainly wouldn't portray Slytherin in a bad light (unlike Ron in PS/SS) and she'd want to be sorted in the same house. I think the story would've been more interesting if Lily Evans, much like her son two decades later, asked to be sorted into Slytherin. Now, Slytherin don't really take muggleborns in, but I think this one exception would've sent a strong message.
Slytherins aren't portrayed as good. While we're told not all of them support Voldemort, we didn't really see it. During the battle for Hogwarts, they deserted the school. Peter Pettigrew is the only known Death Eater who wasn't a Slytherin. Even Sirius, falsely accussed and unrightfully sent to Azkaban, while not a Slytherin himself, comes from an old Slytherin family. Snape did show he was on Harry's side, but he bullied him throughout his Hogwarts years,unable to let go of a childhood grudge against James. He clearly favoured Slytherins and mistreated non-Slytherins.There wasn't a single Slytherin who couldn't at least be disliked.
Lily Evans should've been a Slytherin. The same way Harry chose Gryffindor, she would choose Slytherin. The reason why the Hat considered Harry for Slytherin would then have more to do with his mum being one than with Voldemort. There she'd be in the company of Severus Snape and other potential Death Eaters, including the one and only tragic Regulus Black. She'd probably be the reason why Regulus changed his mind about Voldemort. Having Lily in Slytherin would also explain why she was an important member of the Order: she knew most Death Eaters and had more information about them than anybody else. It would also put Voldemort's offer to join him in a whole new light. Voldemort would certainly be intrigued more by a muggleborn Slytherin than a random Gryffindor. Naturally, he'd offer her to join the Death Eaters in spite of her heritage. Deciding to spare her on Snape's petition would be more understandable.
Some fans (myself included) complain that she's either a Marauder's era Ginny (because of her looks and popularity) or Hermione (because of her academic achievements). Having her in Slytherin would make her totally different from both girls. It would also be more practical as her and James would be Head Girl and Head Boy from different houses.
Having Lily in Slytherin would put more emphaisis on our choices because even though she was surrounded by future Death Eaters, she didn't became a dark witch. She'd be a proof that a muggleborn witch can find friends in Slytherin and that not all of them are Death Eaters. Having a muggleborn Slytherin outwit Voldemort would show Slytherin in much better light. We'd finally have one good representative of the house. Even Ron would be shocked and he'd probably develop some respect for Slytherin.
All in all, Lily was a fitting Gryffindor, but I think having her in Slytherin would do a lot of good for that house in the eyes of the protagonist and his friends, as well as the magical community as whole and of course, us, the readers.