This, I believe would be a very compassionate thing to do. To promote vengeance rather than rehabilitation encourages a spirit that can only lead to further violence in our society. There are many reasons why certain individuals may become capable of doing such horrible things. Among them are acts of self defense, crimes of passion, crimes of hate, crimes of revenge, acts committed by persons suffering a mental illness and of course let us not forget those who are convicted of crimes for which they are completely innocent. It is important to understand the underlying factors surrounding each.
Most people can understand an act of self-defense.
It is called self-preservation. Crimes of passion are committed in the heat of some sort of anger or jealousy. There have been times in our history when this was considered a justifiable act of violence. Crimes of hate are committed against a member of a group, culture or race that the perpetrator holds a grudge against. Crimes of revenge, in which the death penalty is included, is a strike of retribution against someone who has wronged another.