Because we are the line between order and chaos. We have the instincts of the predator but it is tapered with self control. No matter how satisfying it would be for revenge the second we give in we are no longer better than those we hunt and we become as much of a threat to those we are sworn to protect.
So if the subject complies with orders and surrenders we have to up hold the law. If (s)he doesn’t...oh well. Not going to lose sleep.