LONDON — Severus Snape: good guy or bad guy?
If you said “bad guy,” J.K. Rowling wasn’t having it.

The Harry Potter author injected herself into a debate on Twitter of the Defense Against the Dark Arts professor — and regular Potter tormentor — after a user wondered why the adult Harry gave his child Snape’s name.

“Why did you pick Snape to name Harry’s kid after? I’m genuinely curious as he was nothing but abusive towards everyone,” queried @FrazzyJazzy7, a self-described Slytherin who says “Snape is my spirit animal.” (Snape was the onetime head of Slytherin house, archrivals of Harry’s .)

In a long response on Twitter, Rowling gave a full explanation of Harry’s decision: “There’s a whole essay in why Harry gave his son Snape’s name, but the decision goes to the heart of who Harry was, post-war.”

Rowling patiently — and sometimes exasperatedly — explained that Snape, played by Alan Rickman in the “Harry Potter” movies, was not all good nor all bad.

Rowling says Snape, like any other character, “deserves both admiration and disapprobation.”

Still, she couldn’t help but be amused at the emotion the discussion brought out in Potter fans.

“I’ve got to say this: you lot have been arguing about Snape for years. My timeline just exploded with love & fury yet again. Never change x,” she tweeted.

J.K. Rowling explains why Harry Potter names his son after Snape