Lol... your vision of justice is as twisted as a pretzel.letting two disgusting criminals die would have not made a dent in his 'better person' armor. You guys really disagree? Talking about 'it would have lead him on a dark path'? Pls ...
Give me one reason as to why he should let them die - when they are supposed to be killed for his sake - but he already knows that it won't make him feel better.
He would indeed be a failure of a hero, "less moral" as you put it and not the man his companions know him to be, if he did not (try to) prevent it.
Capital punishment is garbage tier in the great book of justice.
The great people of our existence have always told us to not seek revenge. Seeking justice it is indeed the feat of a hero. Seeking revenge is not - and seeking revenge is not seeking justice.
If we must:
An eye for an eye, a tooth for a tooth - even the jews of the past had a better justice system than what you are asking for. If he is to judge let them suffer pain to a similar degree and not above it.
Ryll pointed it out wonderfully
He gets shouty, yells, but accepts the worse punishment of leaving them alive to suffer in ridicule the same way he suffered in scorn.