I'm probably off on this, but I'm as allowed to get in a good WAG as anyone. And I fully expect the same level of scrutiny I show y'all, so bring it on.
Consider the following:
1. Per Word of Jim, effect is as important -- sometimes more important -- than intent when it comes to breaking one of the laws. I.e., if you mean to kill someone with a fireball, but miss, you're impacted less than if you mean to just trip someone with a wind blast and they fall and break their neck.
2. Harry already has Black Magic taint from using magic to kill; possibly not only Justin, but the deaths at Bianca's house.
3. When he cast the Red Court Explode spell, it affected half-vampires.
4. Some of those half-vampires were really old, and their losing their half-vamp-ness caused them to immediately die.
5. Ergo, Harry caused the deaths of humans en masse when he cast that spell.
So, post Cold Days, what are the chances that his sudden murderous tendencies aren't just the Winter Knight mantle -- what if it's also the Black Magic taint from killing dozens of humans with that spell messing with his head?
Discuss, prove me wrong.