A great many violent criminals are themselves the victims of violence and torment. Raised in abusive homes, trapped in poverty, and the likelihood of a sustaining job that can get them out of poverty is extremely slim. When the only option presented to a person is violence, like joining a gang, committing robberies, or dealing drugs, then violence is where they will go. Most times, violent crime is a result of the bad conditions in which a person lives. Human beings are community oriented creatures. We do seek to be productive members of society. We do not seek to disrupt the world around us. Violent criminals become that way because they feel like they don't have a choice.
A truly evil person is one who wantonly hurts others, and doesn't care for the consequences. An evil person does it over and over, often because they are above the reach of their victims. A thug who shoots a store clerk, while clearly doing a bad thing, isn't a fundamentally evil person. These people aren't monsters, and they can be rehabilitated. They want a comfortable life, and want to be safe, law-abiding people. Really evil people, who might actually deserve death are power mongers and tyrants, like Saddam or Mubarak, and callous greedy pigs, like the bankers and business leaders who promote strife and war for personal profit, or who gut the economy and put millions out of work and out of their homes, just to fatten their purses.
A desperate soul, pushed to his last resort and commits a violent act, is not an evil person, and can be redeemed. An uncaring person, who callously hurts others over and over and over, is really evil. The death penalty, as a means to punish violent criminals, does not help. It destroys a life that could still do some good. Only a life that is consistently committed to destruction deserves to be snuffed out.