Did you know that in the mission The Noblest of Men and a Woman, in the duel with Flaco Hernández, if you manage to disarm him during the duel, and then his Sawn-off shotgun, you will be able to spare him. Now keep in mind, he'll probably attack you, and you need to knock him out to complete the mission, but you can complete it and leave Flaco alive.

As you see in the picture, he's alive and well... in a way (because the reticle is red and not white). But alive nonetheless.

Edit: Should note that he's the only one you can do this to (except maybe Calloway but i doubt it), because Granger uses a knife you cannot knock off and Midnight simply commits suicide, and you can't prevent that.