Unbelievable, and sickening, that any of these are allowed. The stats show that killing offenders didn't deter their original offending, and stopping repeat offences (the only other practical point to killing an offender) can be achieved equally well by life imprisonment. At least very few countries do this sort of thing - this quiz shows who.
Revenge killing a.k.a. "justice", deterrent, and the strawman that it's the only way to stop repeat offenses are all poor reasons to execute someone;
recognizing that a person has no value to society and that prolonging their life will only cause suffering to themselves and others, understanding that life is not sacred and that everyone dies sooner or later, accepting that lifelong imprisonment is actually a far worse punishment than humane execution once you take away the irrational fetishistic view of living that most people have, and realizing that keeping someone alive in prison costs a lot of money for no discernible gain ARE good reasons, though.
recognizing that a person has no value to society and that prolonging their life will only cause suffering to themselves and others, understanding that life is not sacred and that everyone dies sooner or later, accepting that lifelong imprisonment is actually a far worse punishment than humane execution once you take away the irrational fetishistic view of living that most people have, and realizing that keeping someone alive in prison costs a lot of money for no discernible gain ARE good reasons, though.