I don't want to incentivise people to start murdering characters; it's not that kind of game. The options are there for people who want to play that way - and in certain cases it may actually be a good idea - but aside from the issue of balancing it out (what could possibly compare to gaining a new fighter and new skills?) it rewards a mentality that's at odds with CK's mostly light-hearted style.
When it comes to certain characters, though (The Chief, Morgana, Tom, etc.) players will, I think, naturally find themselves contemplating their best move of the three options and that's more along the lines of what I want when it comes to the exile/execute options.
Also, it's an adult game and the Boss Girls are cute. You honestly think anything I offered would incentivise 90% of the playerbase to kill them? ;)