So was Martin a real monster at heart, or just way in over his head?
He went from simply being an obsessed and narcissistic wife beater who freaked out over his wife's apparent death to threatening to kill a few people he barely even knew for no real reason. Even if he was a sociopath, it wouldn't mean he was particularly evil.
Yet he didn't even flinch or react when telling his wife he would kill her new boyfriend if she didn't make him go away or when he aimed the gun at his head, and he seemed to have a sense of smugness about him, such as when he decided to make his presence known to scare his wife with his compulsive habits.