Personally, I'm not opposed to the concept of capital punishment...
...there are some criminals who simply deserve to die. However, there are sufficient proven cases of the innocent being condemned to death (especially now that we have DNA testing) that that alone should be all the evidence we need to ban the practice.
I've always found it amusing that the same conservative political philosophy which does not trust the state to regulate business efficiently, run social services wisely or collect taxes fairly, has always been so eager to give that same flawed state the right to execute its citizens.