The antagonist in my story is a sorcerer and has one main goal of taking over a kingdom for himself. He seemingly accomplishes this. He killed the king, banished the crown prince (and by banished, I mean opened a portal and tossed the poor protagonist through into the middle of nowhere), he forced the princess to be his wife, and he dispersed his men through the city to enforce his rule. I'm good with all of that. But! Before he banished the prince, he toyed with the prince a little by forcing a feral spirit into his body, which changed the prince into a shapeshifter-like creature. This, of course, comes back to bite the sorcerer in the ass later on. My question is, does the antagonist have to have some detailed reason for why he did this to the prince? Or can the reason just simply be because he wanted to see what would happen and it was fun?