The answer “because you can” still stands. Sigismund’s soldiers were known to be bringing havoc notwithstanding the allegiance of the poor folk or prospects of any booty. They were doing it simply because Sigismund let them to.
The folk doing that are a bit different from your average Samopše villager in the same way the hardcore Neonazis differ from the average WH team mates. They are primarily driven by their bloodlust and secondary reasons like allegiance/race are used mostly because they are more acceptable to society.
It is the same with criminals on the streets. You have opportunists who want to get something from you, and might hurt you in the process, and then you have psychopaths who want to hurt you, and might get something in the process. Put those people on the background of civil war with their leadership not giving a single s**t and you get groups of psychopaths terrorizing people just for the fun of it (while taking their possessions in the process too).
If the aim of the game is realism and you truly believe that
then you are about to fail greatly on realism.