The four houses of Hogwarts have their own unique traits, but being sorted into them doesn't make one a permanent fixture. In fact, the Sorting Hat struggles with finding the right home for Harry Potter because it takes the students' choices into , and not just their nature.
It could easily be argued that Gryffindor. In other ways, the Potions professor acts very much like he belongs with the Serpent.
Gryffindor: The Sheer Gall
Snape is braver than most people in the Wizarding World, as Harry himself confirms by naming his youngest after him. Unlike most Gryffindors, though, his type of courage appears in the form of pure, unadulterated guts.
Some of his actions include charging into the Shrieking Shack to capture Sirius and Remus, facing Lord Voldemort with the knowledge of what would happen if his double-double-crossing is ever found out, and most terrifying of all, audaciously questioning Dumbledore's actions with absolute confidence.
Slytherin: Extreme Talent
Slytherin is well-known for its focus on magical skill, and, as an extension, expects a high degree of achievement from its . Snape was obviously the finest mind in his school year, suring even Lily, a fact proven by the potion-recipe edits scrawled all over one of his textbooks.
He is an accomplished duelist as well, both in of dark and light magic, but his greatest achievement has to be Occlumency. For someone, anyone, to lie directly to Voldemort without being found out, their mind needs to be sealed tighter than a steel trap.
Gryffindor: Chivalry (Sort Of)
In his youth, before the highs and lows of Hogwarts become a part of his life, Snape protects his crush with a fierce determination, as evidenced in his angry reaction towards Petunia for insulting her sister. Temper aside, he stays close friends with Lily for quite some time, at least until James Potter's arrogance and bullying erodes his confidence further.
However, when he learns that the Dark Lord intended to kill Lily and her family, he tries his hardest to save her, not caring that they hadn't been talking to each other for years at that point.
Slytherin: Slippery As An Eel
Growing up in a broken home (abusive dad, put-upon mom), Snape learns to block his emotions from showing up in any expression he makes. This results in an aura of secrecy about his person, which is why it takes Dumbledore, a man who can read someone's heart in an instant, a while to accept Snape's promises and pleadings.
Although rather serpentine, this sly quality of his allows him to work as the perfect spy — both Voldemort and Dumbledore believe that he's on their side, but only one of them is right.
Gryffindor: Undaunted Resolve
The Gryffindor Lion is a symbol of perseverance, and Snape exhibits this trait in ridiculous quantities. The minute Voldemort goes ahead with his plan to execute the Chosen One, Snape completely shifts his loyalties.
He spends every waking moment assisting in Dumbledore's plan (the only possible way to seek revenge for Lily.) This is best observed in the form of his Patronus, which stays a Doe, the same as that of Lily's, for his entire life.
Slytherin: Expectations Of Purity
The only negative aspect of Salazar Slytherin's House, to be honest, is the demand it places on blood purity. Its are supposed to follow this credo without question, which a lot of them do (but it is unknown how much peer pressure plays a role in this.)
Snape's ing of the Death Eaters partially stems from his own lack of self-esteem, and intense desire to be included in a like-minded group. He may have called Lily a "mudblood" out of rage, but he still uses the term, meaning he understands the full potential of its implications.
Gryffindor: Reckless Behavior
Gryffindors are bold, but sometimes the rush into scenes of danger or make hasty decisions without fully thinking them through (Harry has behaved in this manner on countless occasions.) Snape's recklessness, though, is explicitly un-Slytherin-like, who much prefer to err on the side of caution.
One example of this is his eavesdropping of Dumbledore's interview with Sybill Trelawney, in which she utters the prophecy that sets everything into motion. Also, the creation of the Sectumsempra spell indicates that Snape had possibly used it at some point in the past: a supremely risky chance to take as a student at Hogwarts.
Slytherin: Survival Instincts
Severus Snape knew how to keep himself safe from everyone, a necessary precaution against being branded as a traitor by both the Order and the Death Eaters.
He easily manages to deflect challenges from either side, even going as far as making an Unbreakable Vow to stave off Bellatrix's suspicions. Luckily for him, Dumbledore's trust in his integrity keeps him from being instantly kicked out of the Order (and he would have been if Sirius had had his way.)
Gryffindor: Refusal To Surrender
If he hadn't seen his end coming from a mile away, Snape would have never been able to play his role to perfection. As it stands, he is painfully aware that Voldemort would finish him off one way or another, which finally happens when Nagini fatally injures him in the Shrieking Shack.
He is not deterred even by the imminence of death, asking Harry to collect what he considers his deepest, most shameful memories in order to restore his honor as Dumbledore's devoted servant. Fortunately, Harry reacts with pride on learning that Snape's actions were all geared towards protecting him from Voldemort.
Slytherin: Appetite For Success
Losing his best friend to his hated nemesis is a serious upheaval for young Snape; it could be argued that her (justified) rejection spurred him on towards Dark Magic in a bid to prove his superiority over everyone — but mostly James.
As a Death Eater, his role is shrouded in mystery, but it is clear that his feats of magical strength gradually brought him closer and closer to Lord Voldemort. As twisted it might seem, it is this very desire to advance the endgame that helps him kill Dumbledore, his beloved friend and mentor.
