Whatever justification you need. But remember, the goal is clear, and even if your justification isn't sufficient, you have to go through it. Mercy is inevitably going to backfire. It was practiced for two millennia and it made things only worse (for future generations).
God commanded the Israelites to commit genocide. They failed at one point and now we have jews because of that. Our ancestors failed us in this regard.
And with "you" I mean *us all*.