Legend Of Korra: Will It Bend!

[Nerd-o-rama]

It is important to keep in mind that Aang is still responsible for all of the Fire Nation deaths that occur at the hand of Koizilla. He might not be in direct control, but he knowingly unleashed the Avatar; their blood is on his hands.

This doesn't translate directly to killing them, of course, but it's still something which should be noted.

And he is extremely upset about this at the start of S2 (and I think he brings it up to the guru as well).