KillShot: or Murder Man and M

"He's good at killing but the other guy's bad at dying."

A hired assassin accidentally kills a civilian he believed to be his target. The witness of the wrongful murder can't die. This is a problem for the assassin as the witness wants vengeance and will do whatever he can to get it. Even teaming up with the assassin to stop his employers and a mysterious looming threat from the floor above.