Murasaki got the first taste of what was inside of him when he killed the butcher for her. At first, she was scared. She wasn't certain if he'd turn on her next, but when he didn't, she calmed down and protected Hannibal from the law.
This boy had just killed a man over a petty slight, cut off his head, and brought it as an offering to her ancestral altar. That was more than just a fleeting glympse of what was inside him, that was a front row seat.
But, of course, she didn't let him face the law. He'd get the death penalty. Didn't try to get him psychological/psychiatric help. She became his accomplice because he was her family. Because he was all she had left. Because he did it for her, after all. Because the butcher's petty slight had led to her husband's death. Because she'd lost her family in the war just like Hannibal and understood him.
After that, when he went on his revenge spree, she was still his confidant. Oh, she tried to talk him into stopping, but that's it. She kept his secrets and lied to the police for him.
Speaking of the police, there is Popil. He recognized Hannibal for what he was from the start. He was onto him and knew where the trail of bodies ends. So, what did he do? He had made some token attempts to stop him, yes, but let's face it, those attempts were half-assed at best.
Popil had compassion for Hannibal. He related to him too much. Both had lost families in the war. Both shared the same pain. So, Popil saw too much of himself in that damaged, disturbed boy, and, even though he knew, he didn't do nearly enough to stop him.
Murasaki and Popil were there to witness the early stages of Hannibal's transformation, and they could have done something about it, but they didn't. Through inaction or insufficient action, they let him enter the chrysalis and hatch in peace.
Complacency had as much to do with him growing into what he became as trauma, revenge, or inborn traits did.