I am against capital punishment, myself. Not because I don't believe that there aren't some people who deserve to die, who are a danger to others, who are not human in the important sense of the word. Rather, because I don't believe that other people have a right to determine that another person should die because of crimes committed on someone else. As I believe ARBound said, it's too permanent. I feel it is murder, premeditated, even if the cause is thought by most to be justified. People are too prejudiced by too many things and to put the ultimate life-ending power in the hands of anyone, on a premeditated basis, is actually quite scary with the amount of abuse that is possible, and with the lessening of humanity required to carry it out.
I've tried to imagine what it would feel like if I were on death row for something I didn't do. I've tried to imagine how someone would feel sitting in that cell, feeling time pass, knowing that he or she will spend the rest of their time in this world in that cell and that at the end of it is not the possibility of a comfortable deathbed, surrounded by family comforting you in your final moments, but rather a forced march down a hallway, body trembling, the guards maybe even needing to hold you upright as your head swims, your knees feel like jelly and threaten your collapse. Being laid down on a table, strapped down, helpless, powerless, knowing that there's not a thing you could possibly do to save your life, as spectators watch in smug righteousness and the state officials do their duty with grim faces. The last thing you hear is someone reading off the execution order and giving the command to proceed in a cold, damning voice.
Do I think someone like that jackass that killed those people in the church deserves to die like that? You bet your ass I do! But I think about the innocent guy locked away, awaiting his death hopelessly, as the machinery of the state grinds towards his last moments. And I believe that we're too fallible to know where to draw the line with certainty. I'd rather we put death-penalty-eligible convicts to hard labor somehow, giving them a way to pay for their crimes in some fashion, I don't know, anything but put them to death.
The only time I believe that deadly force is called for is to defend oneself from perceived deadly force.
But I don't know what the right answer is related to violent, horrible people. I don't think anyone does.