Why do you think people should be executed?I don't think we understand enough about ourselves to be able to state that a person is evil, and that consequentially an appropriate course of action is to terminate them. The court/judicial process is also making a judgement on whether something is illegal, not whether it's 'evil'.
You know what I'm getting at though. A serial killer will always be beyond help and locked up for life anyway, so they might as well be killed and some money saved (assuming the system was setup so that it did save money, not like in the US).
