Or using the system used in "A Clockwork Orange"...
More seriously, I'm totally against the death penalty. When a state kills its own citizens, it makes you think of the legitimity of the whole concept of a state.
Yes we all agreed to live together in a society. We agree to obey to the rules. A lot of our prerogatives as human beings is in the hand of the state, but our own life?? no way.
Some say the death penalty helps to prevent crime. I don't agree with it. When one kills another person, he's not calculating what punishment he might get. He just kills out of human passion, hatred, fear, greed, moral deadness and above all failure. And does it really make a difference in the mind of a murderer, spending life in prison, or getting the death penalty?
The USA, for example, have the death penalty in some states, but seem to have a much higher murder rate than most European countries. But, of course, you can say that it has nothing to do with the death penalty, it's a cultural thing, or maybe it has to do with the right to bear arms,...
Of course, statistics prove absolutely nothing.
TBH, I can only give arguments that will only speak to already convinced people. I can't prove that death penalty is wrong. But I still think it's discusting. It neglects every aspect of humanity. An element is sick -> kill it.
An eye for an eye, a tooth for a tooth. Is that justice?