Why not let the cops simple shoot the man once convicted ? Isn't that retribution, deterrence and prevention all in one ? People should know a-priori the conse

Why not let the cops simple shoot the man once convicted ? Isn't that retribution, deterrence and prevention all in one ? People should know a-priori the consequences of their actions. Otherwise it is very likely that there is will be manny shitty prisons and some VIP cool prisons.