***SPOILER*** Based on a true story but as unnerving as any fictional crime drama "Fatal Vows" shows how madness can be instilled in someone at a very young age and fester for years until, like an opened sore, explode and pour out all it's puss and impurities on those who are unfortunate to be around it.
Right away you get the impression that undercover cop Nick Pagan, John Stamos, isn't all what he seems to be. Telling young divorcée Alexandera O'Hara, Cynthia Gibb, whom he meets at a neighbor murder site about his crime fighting exploits Nick impresses her so much in him being a cop and having a livery car service that before you and Alexandera know it she ends up marrying him.
It's after Alexandera is married to Nick that he suddenly turns from a lovable and likable individual into an wife beating and child abusing,against Alexandera's five year old boy Rayn, psychotic. Besides finding that Nick isn't a policeman, in fact he has a long criminal rap sheet, he's also suspected in a number of murders in and around the Bronx section of New York City that includes his own brothers Ray, Tony DeSantis, teenage daughter Jules, Farncesca Scorsone.
Knowing that her husband is a very dangerous man Alexandera tries to leave him together with Rayn. This act of defiance against his authority has Nick get so insane that he ends up scalding Rayan and using Alexandera's fear of him to get herself implicated in her own sons injuries by not reporting Nick to the police.
Nick gets so unraveled as he uncontrollably goes on a killing spree, mostly off screen, against young women in the Bronx that it's only a matter of time before he slips up and the police put the cuffs on him. The only thing that can now keep both Nick's abused wife and step-son from ending up as his next murder victims is if he slips up and gets caught before he murders them. The cops who seem anything but helpful to Alexandera finally get her to put a wire on to get Nick to admit to one of his crimes but something happens that has him do the job for them far more effectively his father Papa Pagan, Ben Gazzara, ending up in the hospital with a massive heart attack.
Nick together with Alexandera, secretly having a police wire on her, go to visit the almost comatose Papa Pagan and it's there when very unexpectedly the truth about Nick and Papa suddenly comes out. The shock of what came out in this hospital visit, with a stunned Alexandra listening in, blew a fuse in Nick's very fragile and delicate mind completely short circuiting it. With the cat now out of the bag, about Nick and Papa Pagan's secret relationship, Nick went into self-destruct mode giving his automatic handgun to Alexandera to finally put him, and possibly Papa Pagan, out of his misery. With the cops bursting into Papa Pagan's hospital room Alexandera can only watch and put Nick's gun away leaving it up to the law, and a battery of court appointed psychiatrists, to decide Nick's fate in court.
P.S According to the movies epilogue Nick Pagan got 75 to life for the murder of four and possible five other victims. Nick's father Papa Pagan died in the hospital while under intensive care of a heart attack the day the verdict was handed down against his son.