"Cruel deeds by dark and light"

[Lord Melkor]

Well, it would seem to me that the cruelty of a deed depends on the deed itself, and not on the person it is inflicted on. If the Dwarves would've slowly tortured captured Orcs to death, or perhaps executed them by setting them on fire, then it is quite obvious that those deeds were cruel, even though the victims were 'just' Orcs. The Dwarves would've done well to remember that they need not stoop to the tactics of the orcs, lest they become as cruel as the orcs themselves.