here's a reason. THEY ARE MONSTERS! they do as they please. they went into winhill because that's what they do. they kill and take over. it's THAT simple! granaldo was a monster in the training area that was hardly ever seen, the oilboyles were there because im pretty sure that shumi guy put them there(or they were just little monsters to start with and lived in the oil and grew after the how many years that that place was closed down). Also, Abadon is a monster as well. If u didn't notice, the great salt lake was full of Vysages which were practically undead(or whatever u want to call them) and the Abadon was just a really strong monster that lived there. U think square put in monster bosses because they were possessed? I think not!
Nothing of that make actually sense.