Consider the following death penalty case: the adulterous woman (Jn. 8:1-11). Would you have wanted Jesus to kill her? It would’ve been justice, because He could’ve killed her, but it wouldn’t have been mercy. He chose to be merciful, and in doing so gave that soul time, and possibility to arriving at repentance, and holiness, if she wished to reach them. He led by example the following teaching: “Be ye therefore merciful, as your Father also is merciful.” (Lk. 6:36). Merciful to whom? He made no distinction between sinners.
Therefore, no matter the situation in a death penalty case, do you say mankind should or shouldn’t follow Jesus’s example of mercy?