The death penalty should ALMOST never be used. The only use for the death penalty is for world leaders that direct their subordinates to commit atrocious acts.
Normal civilians, no matter how dangerous, should only ever be treated with dignity. There is no place for state sanctioned murder.
A) It is immoral.
B) The justice system isn't perfect, and death is final.
C) The actual cost of going through all the trials and effort to put someone to death is typically higher than just keeping them locked up.
D) There is no humane way to put someone to death.
E) It is not effective at preventing crime. It only makes it so people have nothing to lose by being caught.