If it looked like it was in pain, I'd have my boyfriend do it. I think that knowing when to end an animal's suffering is part of the responsibility of taking care of them. I personally am too much of a wussy to do it myself, but I let a fish die on it's own before, and I really regret it.
Just over the weekend my friend accidentally ran over his brother's dog. The dog was not going to make it, and I won't go into it graphically, but will say that it was a mess
. He had a choice to love on the dog, tell him he was good, and shoot him in the head, or let him lay there and die. This dog was very loved, and he shot him. I thought it was one of the most humane things that could have been done.
Sad, but humane.