wow...sorry to hear that this has happened.
Being a dog owner myself, I don't think I would do anything. That might sound bad, but I love my dog and if he killed another dog, that sucks and I am sure he would then get his ass sent off to obedieince school, but I would not put him down. I think killing your dog because he killed a much smaller dog is not an act that warrants the death penalty. If he attacked a small child, so be it...I would shoot him myself. People always come first. But it could have been a case that the big dog was playing/fighting with the smaller dog and never knew its own strength and killed it by accident.
If you could prove it killed the little dog out of sheer verocity, then that may be a different story as well.
This is an interesting thing because I have wondered about this situation myself. As many know in ligh of yesterdays lab vs. pit bull war(still funny by the way), I own a chocolate lab. He is 22 weeks old and is well over 50 pounds with a lot of growing yet to do. My brother on the other hand owns a miniture pincher. This dog is the runt of the litter too. He is over a year old and weighs 6 pounds. Now, these two dogs love each other, but Bubba(my lab) does not know his strength and if we were to leave the dogs alone I am sure he would accidently kill the little min. pin. I mean, these two guys are funny as hell to watch. They chase each other around and all sorts of funny stuff happens, but that said, Bubba has sent the little guy flying across the room a few times by accident. Its kinda scary....so yeah, I sympathize with the problem.
Do whatever your gut tells you to do. In my case, I can not imagine killing my dog because he killed my brothers dog...but then again you don't really know what you would do until you walk in those shoes.
good luck,
Sean