The car thief/murderer did not do this in a vacuum.
Shooting him will only take out the visible symptom. He has a people around him, maybe gang members, fences, suppliers of his drugs, etc. It will take more than just shooting him. I am, of course, not advocating anything illegal, just pointing out that shooting him alone is a partway measure.
I meant that I want the legal system to take care of this right away, not years from now.