The idea that you have to "take prisoners back" is absurd
"Back to what?" exactly? A judicial system where the local lord's half-wit brother holds sway? Why would "Lord Mucky Muck" care what happens in a neighboring area (where the evil thing SHOULD HAVE GONE) and what is going to be the outcome?
As most parties have character more intelligent AND good, they are more than capable of "meeting out justice" even if that means death.
I love the idea that you take all these "evil characters and monsters" to some LG society where they lock them up. Sounds like a great day for an evil power to "pack the jails" and release all of that evil for a night the population will NEVER get.
Sorry, but "Lawful Stupid" and "Stupid Good" are not alignments, those are death sentences for the majority of people living in the area