So at the end of the main quest
when your given the optional choice to free Mannimarco from his imprisonment by Molag Bal
Why is this an option? The guy killed you, thousands of others, clearly doesn’t show any sign of repenting or changing his ways, all your allies are satisfied with him in this state. What reason would you have to let the guy go?
I did it on my character thinking that it must have had some pay off like he’d try to fight you (for a satisfying third beatdown on him), or some divine force would strike him down, something to rationalise the decision so picked it out of curiosity. Turns out its a "what did you think was gonna happen?" situation, he mocks you for it, swears revenge, Tharn questions your sanity and Lyris gets a "bad feeling" about the decision.
Instant regret, since it wasnt a character-driven decision, just picked it out of player curiosity. Yeah he escapes either way, but surely a good character wouldn’t let a mass murderer walk freely into the world again.
The question is why would any of your characters release him?