Destroy & Become
There are two ways I could go about this, I could eliminate those who I personally feel deserve it. Those I have met and despised, but that would be selfish because I don't believe I have ever encountered true evil. My other option is democracy. I could take suggestions and allow others to vote. But it won't be a true democracy I will still have the power of the ultimate decision, otherwise I would just be murdering annoying celebrities. Much as I would love to never see or hear of them again they aren't deserving of this fate. It wouldn't be anyone in prison already answering for their crimes. It would be those who had escaped justice. Possibly those unnamed individuals who we only know by their evil deeds. As for how it would be accomplished it would all seem accidental. The deaths would look like divine justice, suitable for their crimes. That way I could keep my actions secret and continue this work in private.
This responsibility would be a terrible burden that would likely wear down my soul. I'd have to guage levels of evil I don't even want to know exist. The power would come with a duty to help those who would be hurt if I chose not to use it. In the end I would become what I sought to eradicate, an evil being with bloody hands and a twisted mind.