I generally agree but there are costs to not executing someone clearly guilty as well and it's about measuring these costs. For a general murderer - sure the costs of keeping them banished forever are quite low but for someone like Musolini or this mayor? There are real costs of keeping them alive in banishment primarily the risk of them coming back or leaking influence back into the real world so imo death sentence here could be justified.

Though in practice I agree that it's safer to not give government this power as overall risk of abuse is too high but ethically it's completely justifiable to kill someone who's incredibly dangerous even if 100% certainty is not possible.