Nothing.
There's nothing in the world I would want that could make me kill a human being, and I think anyone actually willing to kill someone over stupid things like a TARDIS or having magical powers is an awful person.
Of course, if we focus on the less egoistic things, if my wish would be to for example end world hunger or something like that, killing someone might be worth it. Whether I would actually be able to do it or not is a completely different question.