I think it’s pretty well established that he doesn’t personally have control of his actions in the avatar state, at least at that point in the story. He doesn’t even know how it works. Thats a pretty major plot point. Combine that with him also being merged with the twin spirit of the one that was just killed, and justifying an argument of “but what about all those soldiers” really just falls flat.

I’d say the closest he came to killing of his own volition was when the sandbenders stole Appa… and he still didn’t pull the trigger.