Revealing mistake: It's a stunt double when "Ray Liotta" falls down the stairs at the end, just look closely at how different his face is. You can even play it in slow motion, and it becomes more ridiculous. (01:45:15)
Other mistake: Penny comes into Karen's room to check on her, and Karen is surprised to see her there. So Penny obviously let herself in somehow, but we see in the next scene that the alarm is on, and Penny does not know the code. If she does not know the code, there's no way she could have got into the house without Karen knowing. Even if Penny had a key to Karen's place, she would have set off the alarm.
Continuity mistake: When Ray Liotta throws Madeline Stowe on the bed after she pulls the gun on him, he used his hands to pin her arms down. Then they cut to a different angle, her arm is pinned under Ray's leg and he's grabbing her hair.
Continuity mistake: When Ray Liotta is threatening the burglar, a door on the left side of the screen is closed. In the shot when Liotta says "he wants to kick your ass!", the door is wide open.
Factual error: When the narcotics officers raid Michael and Karen's home on suspicion of drug dealing at the residence, tipped by an informant as stated by one of the officers, that would not be probable cause to raid a home. They would have had to do extensive surveillance and spotted many people coming and going, or witnessed the dealing itself, to do a drug raid, even if it was actually set up by Pete.
Continuity mistake: When Ray Liotta is lying unconscious at the end just before getting shot and killed, the blood pooling under him changes volume and position once and then back.
Answer: Pete was a psychopath and a murderer who did not think or act in a logical, reasonable, or restrained manner. He had become so enamored of Michael and Karen and their affluent lifestyle, that he went to extreme and dangerous lengths to ingratiate himself with them. He was not at all rational.
raywest ★
And also, his years of being a patrol cop and seeing the brutality of society on different calls may have made him snap. I mean, look how heartless he was-he throws a naked woman out into the street in a dark alley and leaves her there after having sex with her in his patrol unit, no regards for anything or anyone at all.