The Grim Calculus

So Im writing a story, and I need to explain to someone why letting a bad guy go is desirable.

Subject A has a 99% chance of killing 1,000 people, and a 0.1% chance of saving the lives of 1,000,000 people. Since the net lives is +10 (.99*-1000=-990 + .001*1000000=1000) letting him go is the right choice, morally and rationally. Or am I not understanding how the logic behind such a decision is supposed to go?