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 .)

After Rowling explained that Snape had died for Harry — and Harry’s mother, Lily, whom Snape had a crush on — some Dementor-like sourness descended, and continues to do so.

“Nothing Snape does is redeemable,” tweeted @BeACartoonHeart.

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

“Snape is all grey,” Rowling tweeted at one point. “You can’t make him a saint: he was vindictive & bullying. You can’t make him a devil: he died to save the wizarding world.”

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.

Next up: Potter fans take on “Paradise Lost.”