If you're robbing a place and your punishment for being caught is effectively life in prison, what reason do you have not to murder all the potential witnesses

If you're robbing a place and your punishment for being caught is effectively life in prison, what reason do you have not to murder all the potential witnesses to improve your odds of getting away with the crime? All you have is morality, and I don't want to rely on the morality of criminals. In Heat, Al Pacino's character summarizes this attitude in a quick line (paraphrase): "Once it escalated to a murder-one for all of them, why not pop guard number two because ... what difference does it make?" The scene is an armored truck holdup and a loose-cannon member of the robbers' crew shoots one of the two guards (By accident? Or is he a psychopath?). After this, the second guard needs to die, as a living witness, because the entire crew is already exposed to first-degree murder charges. Simple risk management.