One big reason I support the death penalty for certain crimes (where >1 person is murdered, where mutilation/torture/rape is done to the victim while alive before killing them, or for children) is yes, for some evil acts, retribution is in order, as is closure. While it's true that the practice of the administration of the death penalty (who gets it vs who doesn't) has racial disparities that must be fixed, that’s a reflection of the justice system’s current flaws, not the death penalty itself.
Another reason, not often mentioned much is the danger to other prisoners these psychopaths pose. Dylann Roof would probably be highly sought after by the Aryan Brotherhood. Other mass murderers and psychopaths are good fits for prison gangs, especially when as lifers, they got nothing to lose. It's kind of hard for other prisoners in the general population to be rehabilitated if they have to kill and stab others to avoid being raped and/or killed by lifers with nothing to lose or their henchman.