I haven't seen anyone say he planned this for years. I do believe that his behavior with Mollie was that of a predator.
I have a narcissist/ sociopath/psychopath (depending on what therapist we might ask) in my family circle. She looks 'normal', is viewed as a wonderful member of the family of Christ in her own circle, and was once called Wonder Woman because she's allegedly just endured so much. The truth is, she flies under the radar and is guilty of complicity in a death where she was viewed sympathetically by LE, she is spine-chillingly psychologically violent behind closed doors, she is an abuser of children, and she is a thief. And so much more. The chances of her ever getting caught are slim to none. I would describe her as a monster. The monster who lives next door who smiles and waves.
I don't know what brand of 'monster' this guy is but tragically, Mollie did before it was over. And I don't think he just suddenly became capable of this. People can be horribly depraved and lacking in empathy without appearing to be monstrous.