It's interesting how Evilus ever got that many members. What kind of an adventurer signs up to be a faceless minion trapped in a dungeon, pretty much unable to do anything but follow orders, which often lead to a certain death? Maybe they are all drug addicts or something. Monsters and a few key figures seemed to be the main army anyway.
If lots of monsters start pouring into the city, it's not like random, otherwise uninvolved adventurers would just keep sitting on their hands. It would be quite unreasonable, seeing how it's the adventurers' job to hunt monsters, plus they also need the city functional. Not that Evilus would care, and for Finn they would be free, if uncontrolled, reinforcements.