I believe you did the right thing. As I read your story, I think I would feel the same way you do...not because I took an animal that was already wounded, but because the " less than perfect" shot you hit him with didnt end his suffering immediately. Maybe you are a little down because you feel like you added to his suffering a little before you ended it? I agree thats a little hard to get over. I mis-hit a deer once in my hunting career (with my muzzy as a matter of fact) and I felt like crap over it for a week. I think the good news here is you DO feel a little bad about it. There are a lot of jokers out there who dont care at all. And some even get a kick out of making an animal suffer, so consider yourself a good, ethical hunter. The crappy feeling just reminds you to keep working toward perfection.
God bless and good luck in the future.
SC in LR