I'd immobilize the guy first, allowing him to watch as I slowly and painfully kill his child. Then I'd give him a few minutes to think about whose wrath he invoked with what he did.
And when he realizes he has lost everything, and asks for a merciful death, I'd take a couple shots on his knees, to make sure I hit an artery, and then I'd leave him to bleed out in pain while I walk away.
Imprisoning him for life in a cell over which I have complete control and where he had absolutely nothing, not even a bed, where he'd only get the daily food and water doses I'd give him would be better, but I'd need to have such a cell already built in the first place.
...Now I just need to find someone with a 12-year-old child willing to do something loathsome to me.
And then I shall unleash my revenge on him thoroughly.