triggerman

1. An underworld gunman who in premeditation shoots a victim.
2. A gunman; a shooter: "Where is the moral justification . . . in offering the platoon survivors-the actual triggermen-immunity?" (Nelson DeMille).

noun

One who murders another: butcher, cutthroat, homicide, killer, manslayer, massacrer, murderer, murderess, slaughterer, slayer. See help/harm/harmless.

The noun has one meaning:

Meaning #1: a professional killer who uses a gun
  Synonyms: gunman, gunslinger, hired gun, gun, hit man, torpedo, shooter