The primary reason people in USA support the death penalty is out of fear that heinous criminals will be paroled or freed or somehow be put in a position to escape, and that once out, they will reoffend. This has unfortunately happened, especially prior to about the 1990s. If people can be guaranteed that Joe Serial Killer will actually be locked up and the key thrown away and he won’t be coming out again, then they usually don’t insist that he be executed. This isn’t just my opinion as I used to do a lot of anti-death-penalty work and attended conferences, talked to experts etc.