Killing a puppy would hurt me a lot.
I remember being a little kid and absolutely hating my father for drowning a litter of 6 puppies one year. I thought he was a cruel bastard.
While I'm older and understand that we couldn't have afforded to keep them all, I'm still angered by the fact that he didn't just have the dog spayed and prevented that heartache.
Killing a puppy would make me feel guilty, whatever the circumstances. Even if I was a vet putting an animal to sleep to alleviate their suffering, I'd still wish for another, better option.
That said, I would kill a puppy for a million bucks. I don't think killing the little fella would be right, but I think not giving my kids a shot at college and a better life would be even more wrong.