Until the end, Grandpa Roland chose not to reveal anything to Marceline except for the fact that her parents' death had something to do with the werewolves.
He pretended to be unwell and sick so that Marceline would not ask further questions. But he promised her that he would tell her everything once he was ready.
As much as possible, Grandpa Roland wanted to keep her safe by hiding the truth from her. The more she knew about werewolves and vampires the more her life would be in danger.
She was not yet a gatekeeper so she didn't have the protection against those supernatural beings. Grandpa Roland never wished for her to get hurt.
He had sworn to Marceline's parents that he would protect her. He hoped for his granddaughter to live a normal but happy life away from the dangers.
