Maybe you should tell the instructor and see what they think about it, I don't think they would ban him from the classes. Of course dogs think so much differently than us.. he may feel bad for it or may not understand, but I don't think dogs do things to intentionally harm anyone or anything the way it may seem. Of course he shouldn't be put down! Just be more cautious when taking him out.. give him more attention. It may never happen again. I'm sorry it had to happen and I really hope the other dog is alright.. No animal is ever a lost cause, just remember that and it may help.