In that very likely circumstance, I’d ask the wizard why he didn’t just go to the police and tell them where to find the guy. Or is this wizard just being a dick?
Why you gotta hate? 
Don’t fight the hypothetical. That’s not the purpose. Otherwise, why not just point out that there is not and cannot be any such thing as this magical killing box?
In that case I’d tell the wizard to press the button himself.
If that’s still fighting the hypothetical, then I’d still throw the box into the sea.
You know thoughts pop unbidden into your head when you least expect it? Like when you’re having sex, and suddenly out of the blue you’re thinking about doing it with your ugly Aunt Lulu?
Yeah, too risky to use a box like that. What if I killed someone that I didn’t mean to kill?
Destroy it, sez I.
I must say that no, that has never happened to me. Her name isn’t even Lulu!
You know these magic thingies never work that way. You’d kill Bob, and then facepalm when you realize that his cancer is now dead as well. Killing Bob was the way it killed the cancer.
Cue Twilight Zone music.