Something along the lines of "Ahhhg...it...it..hurts...." or *Breathing heavily* ...why...why me? or "...not...not yet..." *Gushes up blood*
I think it would add some realism to the game.
Would really make you think before killing someone in the game.
I noticed in Skyrim how death isn't really treated like a big deal and I think this would add some much needed emotion to the game.