Well, all of you who have seriously answered that they'd kill others for political motives (e.g. people who are cruel to animals, corrupt politicians, etc.) are potential "monsters" just like Stalin or Hitler were. Especially the one talking about killing those who harm animals. Even if he didn't mean what he said, there is no doubt there are crazy people like that out there who would use that power to murder people they disagree with.
But, I don't think "not murdering" is the answer either. Those people saying they wouldn't use it to kill anyone are just indecisive and don't have a strong highly reasoned moral ground, in my opinion. People who harm other humans in very significant ways do deserve to be offed, even if they can't control their actions and even if you don't believe in free will.
So, what would I do? The power is too strong for one person, but too useful to be ignored. In principle, I think this power should be shared and in control of some entity whose main goal is to look out for the safety of others. So practically what I mean is I would create some company (which may or may not work for some government) of people, and the choice to kill somebody would have to have reasoning and evidence behind it. The people in the "company" would then vote for or against killing a certain person. I would choose the people who make this "company" up based on their use of reason.