If your protagonist is intended to be a murderer, they should murder people; if they're not, they probably shouldn't! It seems so obvious, but it's surprising how many shows it trips up. Merlin is another; Merlin is so happy to kill people who threaten the kingdom, and the show is so happy to ignore the fact that its loveable protagonist just killed someone. I'm not saying your protagonist can't kill people - in the case of Patrick Jane in particular, I thought it was a great development, both fitting for the character and extremely interesting - but it would be nice if the show at least recognised the weight of it.