How can Killian’s heart be pure red?

I agree it shouldn't be pure. I mean he did technically cause BlackBeard to die, by making him walk the plank. So in a way he DID kill someone.

Well Hook was never as evil as Regina was and i don't think he's killed as many people. I'm betting his heart must've darkened a little bit after killing Blackbeard and it just wasn't shown but I think the things he's done since then has helped restore the light. Plus who knows? We haven't seen Regina's heart since getting it back from Zelena so there might be some lightness returning to it. Maybe she'll take it out again thinking taking it out will make her immune to The Snow Queen's curse so we'll see what its current state is. Anyways yeah I think Hook was a more easier villain to reform so his heart must've gained light a lot easier. Then again, we don't know how many people he's sent to the plank or stabbed with his sword. To tell you the truth, I don't really have a definite answer. I'm just stating what's possible.