Why don't we just kill off all criminals?

I mean, seriously, I am tired of my tax-dollars to spent feeding, clothing, and sheltering the parasites of society. Look at the rehabilitation rates of all criminals: murderers, rapists, thieves, assaulters, financial criminals, etc. When they come out, they just do the same crime over again and go back to prison and know the routine. Why don't we just kill them all off? As the saying goes, "a cheetah never changes its spots". I know we are going to kill many innocents as well, but that is the trade-off to reduce crime and to put my tax-dollars to better use. If you are accused of a crime in the first place, you weren't that much of a good and contributing member of society to begin with.