1) we are human beings. nothing we do is, or can be, perfect. hell, even jesus was a sinner. Or, in a more modern context, "everybody fucks up".

2) there are evil people in the world. Even people who arent evil themselves either have moments of malice or just follow orders of people who are malicious or evil.

3) even if the above two werent true, there is the 'problem' of free will. as in execution, you may act violently without malice or evil desire if you beleive you are justified. Key word being beleive. you may be misled into thinking you are justified when you are not. canada, for example, might invade america if they thought we were executing canucks en masse, nazi-style, when in fact they just picked up a garbled rerun of some history channel show.