The death penalty is absolutely justifiable in cases of rape and murder that are proven beyond a shadow of a doubt to administer justice to those who deserve it. A death spell exists for the same reason, especially if these types of people get off scot-free.
What I dislike is the "humane" forms of administration, such as lethal injection, as the criminal deserves a violent horrifying death in the same capacity that they administered to their victims. Disposing of them in this way is a mercy and a kindness almost, although i have heard in some cases leathal injection can cause excruciating pain. This isn't because I would want cruelty for the sake of it, but to administer justice in proportion to the offense. If I had a loved one who was violently victimized I would want the very same things done back to the criminal, to make them feel the pain and horror they themselves inflicted.
The judicial system should allow family members to choose and even to personally carry out a method of punishment for these types of crimes under lawful supervision. This would be the equivalent of cursing someone to death because they raped or murdered one of your family in the way you saw fit.
Disposing of someone out of an act of revenge should also not be punished in the case someone is avenging a loved one who was victimized by said type of violent crime. I knew an older guy I worked with on a job that was in jail for almost 20 years because he disposed of the person that he literally watched shoot his father when he was 9 years old. He wanted revenge for his father and took it when he was older, around 15. The police even told him they sympathized with him but had to process him anyway according to the law. The court knew of the circumstance and softened the sentence according to this, but he should not have had most of his life wasted just for rightfully avenging his father. Justice was served by his hand and he was punished.