I'm not defending WHOM they killed, but rather than WHY.
You send a bunch young kids to a training center designed to make them use to taking orders and good at killing.
They will kill people when you order them to.
When they "raze" cities they give insane amounts of warning to come get captured instead. Days ahead of time.
Some idiot somewhere thought they didn't have to listen and this tragedy happened.
Obviously the whole entire war over there is silly and stupid.
But it's not like these kids were genuinely going "I can't wait to shoot some children!" They were probably thinking "Oh God someone is in there, are they going to kill us all?"