Yeah sociopaths and narcissists have a way of making you trust them, that's part of their "thing".
I think you're spot on.
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I believe MP has been living and cultivating a double life for a very long time. As he grew up under less than ideal circumstances (parents abandoned), I believe he became an addict at a young age--I think some sort of sexual addiction. I think this explains his surface level friendships with his peers (or lack of friendships) and frustration with women. I think he poured himself into securing relationships with women because that's what he "needed" to be happy, but once he got in to those relationships he was unable to reciprocate the kind of love that he craved. He was an empty shell, always craving more attention from from women to fill the void inside. Physiologically, his addiction altered his brain as it was overloaded with dopamine on a regular basis. He became depressed, moody, and irritable--unable to even get back to a normal level of happiness or pleasure.
His strict religious upbringing solidified the divide between his real life (uncanny_matty) and his alternate life (marty_radical) and amplified the guilt, shame, and resentment he felt. If his friends/family/employers knew about his second life, surely he would lose everything. Sealed in secrecy over time, his life became more compartmentalized. Narcissistic and sociopath tendencies became more apparent as he became more and more needy for escapes from reality.
Ultimately, split between two starkly different lives, moral codes, and personalities, he no longer knew who he was. He literally began striding down the path towards insanity as he could not reconcile these two identities.
Enter LP. Bright, happy, pretty, as he was "checking her out" on Instagram, he felt that--Here she was! Here was a girl he could finally be happy with. As long as he had her affection, things would be ok. That marty_radical guy, that's not even the real me... so he turned on the uncanny_matty and dialed up the charm. Not because he loved her, but because he craved her love and what she could do for him. Honestly, he felt this way about a lot of the girls he met online, but she actually reciprocated. He did everything to get close to her, to get her to adore him. He had perfected the skill of becoming anyone he thought he needed to to be to get what he needed out of another person.
After the wedding it all came crashing down. MP thought that LP would fix him, but she was really just another fix. He was over her in a week, but continued to use her. And he continued to "check out" and possibly communicate with other women. Poor Lauren eventually becomes aware that something isn't quite right. I don't think she ever found marty_radical, but I think she was aware that uncanny_matty wasn't exactly the guy he had pretended to be before the wedding. As LP's novelty wore off, he became indifferent to her. Eventually he resented everything about her, because she became a barrier to what he thought he needed to feel good. And she made him feel guilty. Intensely guilty. She was everything the opposite of him. Truthful, open, genuine. She truly believed in God, she gave her time to her church, she loved generously. In the brilliant, pure light of LP, MP shriveled into the dark. And he embraced the dark.
In the final months leading up to that night, MP dove headlong in to marty_radical (I believe this was triggered by some sort of event--Possibly a discovery by LP). After all, marty_radical was the true MP. uncanny_matty had become the "fake identity " a long time ago (possibly before the first marriage). His anger and rage were extreme. It was her fault! She had ruined his life. She made him feel this way, and with her out of the way, he would be free to be or do anything he wanted. He hated her, hated God, and hated himself. marty_radical intentionally killed LP out of rage and resentment. In that moment he meant to destroy LP, his previously girlfriends, God, and uncanny_matty. After all, it was these things that were keeping him from enjoying life as marty_radical, the real MP.
This is all opinion/conjecture. I do not have insider information on this case--I only have a strong feeling that some elements of this narrative will be proved true in the next several months.