tall enough to meet the sun

O, I AM FORTUNE'S FOOL!

He could have been a notorious Death Eater with his skill in dueling and spell-creation, but he wasn't. Sirius didn't know he was a DE, Karkaroff didn't accuse him of any specific violent crime, Bellatrix accuses him of deliberately avoiding the action, and he didn’t kill anyone before Dumbledore. He was relegated to being a spy at Hogwarts by Voldemort, which meant he probably had a relatively clean record (and imo, I don't think Snape revealed the full extents of his talents to Voldemort in the First War).

In PoA, he restrains Lupin without physically hurting him in the Shack, despite thinking he's working with Sirius. He threatens to give Lupin and Sirius to the Dementors, but when given the chance to do that when Sirius is unconscious, he takes him and the Trio back to Hogwarts (he does want Sirius Kissed, but he also wants Dumbledore's and the Minister's approval). Compare that with Sirius and Remus, who were going to kill Peter in cold blood in front of three teenagers, even though Peter's testimony could clear Sirius.

In OOtP, when he and Sirius are arguing, Sirius draws his wand out first.

IIRC, there are only three times we see Snape using magic to hurt someone potentially seriously. 1) During SWM when he cuts James's cheek. This was self-defense after he'd already been ganged up on, hung upside-down, and choked with soap. SWM also happened after the Prank, so he thought Sirius (and James) meant to kill him. 2) Avada to kill Dumbledore. This was a mercy kill. 3) Sectumsempra, aimed at a DE but accidently hit Fred. This was him trying to protect Lupin. Dropping the branch on Petunia was accidental magic, he only used Expelliarmus (as opposed to something nasty) to knock down Lockheart, he only gives Harry a magical bitch-slap when Harry's stupidly chasing after him and the other DEs in HBP, and during his fight with McGonagall he was trying not to hurt her.

He's not just non-violent, he's non-violent compared to other "good" characters even when he has the ample opportunity (and probably the desire) to be violent. He invented hexes as a teenager, but no one (not Lily, James, Sirius, or Remus) ever accuses him of hexing people for fun like James does, and he's not on the detention cards either. He probably did use his hexes against the Marauders, but as self-defense when they attacked him first.

He was the victim of extreme physical bullying courtesy of the Marauders, his dad likely physically abused him and his mother, and he's surrounded by DEs who get tortured as punishment by Voldemort and also torture/murder their victims. 

It's also worth noting that although he personally developed sectumsempra as an incredibly violent spell, he doesn't appear to have shared it with anyone else and only canonically uses it once with the intention of hitting a DE.

He is capable of incredible violence, and perhaps an urge toward it is what led him to the DE in the first place and also inspired him to invent sectumsempra. But despite his capability for it and the evident temptation, he chooses non-violence.

― Ocean Vuong, On Earth We’re Briefly Gorgeous

“I have found both freedom and safety in my madness; the freedom of loneliness and the safety from being understood, for those who understand us enslave something in us.” 

― Kahlil Gibran, The Madman

― Virginia Woolf, A Room of One’s Own

“All freedom is relative—you know too well—and sometimes it’s no freedom at all, but simply the cage widening far away from you, the bars abstracted with distance but still there, as when they “free” wild animals into nature preserves only to contain them yet again by larger borders. But I took it anyway, that widening. Because sometimes not seeing the bars is enough”

― Ocean Vuong, On Earth We’re Briefly Gorgeous

― Ralph Ellison, Invisible Man

“Freeing yourself was one thing, claiming ownership of that freed self was another.”

― Toni Morrison, Beloved

― Jean-Paul Sartre

The caged bird sings with a fearful trill of things unknown but longed for still and his tune is heard on the distant hill for the caged bird sings of freedom.

― Maya Angelou, The Complete Collected Poems

― Franz Kafka, Amerika

“Between stimulus and response there is a space. In that space is our power to choose our response. In our response lies our growth and our freedom.”

― Viktor E. Frankl

― Fernando Pessoa, The Book of Disquiet

“Freedom is the will to be responsible for ourselves.” 

― Friedrich Nietzsche, Twilight of the Idols

― E.B. White, The Trumpet of the Swan

“This tremendous world I have inside of me. How to free myself, and this world, without tearing myself to pieces. And rather tear myself to a thousand pieces than be buried with this world within me.”

― Kafka Franz, Diaries, 1910-1923

― John Fowles, The French Lieutenant’s Woman

“And I want to be held down. I don’t know what to do with the horrifying freedom that can destroy me.”

― Clarice Lispector, The Passion According to G.H.

“Against Interpretation” by Susan Sontag

“Against Decoration” by Mary Karr

“The System and the Speaking Subject” by Julia Kristeva

“Word, Dialogue, and Novel” by Julia Kristeva

“Walter Pater’s Eucharist” by Benjamin Taylor

Conclusion to The Renaissance, Walter Pater

“On Beauty and Being Just” Elaine Scarry

“Transmission & the Individual Remix” by Tom McCarthy

“The Laugh of the Medusa” by Helene Cixous

“Sesame: Of Kings’ Treasuries” by John Ruskin

“The Nature of the Gothic” by John Ruskin

“Tradition and Individual Talent” by T.S. Eliot

“Days of Reading” by Marcel Proust

“E Unibus Pluram: Television and US Fiction” by David Foster Wallace

“The Art of Fiction” by Henry James

The Pound Era by Hugh Kenner

Real Presences by George Steiner

The Art of the Novel by Milan Kundera

The Art of Recklessness: Poetry as Assertive Force and Contradiction

Heroines by Kate Zambreno