A paradox:
Lenient, perhaps, but not in the same way as I think you suggest, Kelkat. Sometimes the law is arbitrarily harsh (three strikes rule, for example). The problem more resembles that of inconsistent treatment due to the fact that mediating human behavior is simply...very complex.
That the death penalty should be used for people who are considered to be unsustainable is consistent with a philosophy of socially oriented utilitarianism. It can't be a perfect measure and there are many cited cases of the death penalty being wrongfully imposed, and it seems often said that this is often influenced by discriminatory measures such as racism. Nonetheless, I do believe that as a principle, the death penalty can be necessary in the absence of any other reasonable alternative.
In the interests of fairness, the appearance of the legal system, with its prescriptive sets of rules, however promotes the notion of morality being similarly prescriptive. IMO, this is counter-productive, and so I would think that such things as the death penalty should only be considered if one were to actively refer back to the intentional, consequentialist outlooks on morality that the legal system are inherently run by. We must remember that the laws and sentencing guidelines are just that- guidelines, and they must exist for to use a system without those would be to place unwarranted trust in the hands of the executors of justice.