he killed his father before he was possessed.
He became possessed after taking a toy dragon into the castle treasure room to play, and finding the Sword of Lies. I'm pretty sure that happened before he had his father assassinated. He undoubtedly was a spoiled, self indulgent brat to start with, but probably not a power hungry sociopath who had the intelligence to execute a plan to become the duke, and then go play with a child's toy.
