Stupidity: Abel sends a video next door showing Chris being sexually assaulted by a stripper at the bachelor party, which would actually get him in a lot of trouble as the video shows him encouraging it. Yet, for some reason, Chris and Lisa don't bother to use this important piece of evidence to make a case against his harassment of them.
Stupidity: At the end of the film, Abel had nothing to gain from lying about having a gun to the police officer. He had no way to prevent Chris and Lisa from presenting the informant's cell phone and exposing his involvement in the break-in and vandalism of the couple's house. Additionally, testing from the shots fired in the house and the car would prove that there were two different guns involved. As a long time police officer, Abel should have known that giving up the gun would have been the easier way out instead of lying about it.
Answer: He already had a much bigger mental problem in general, and not just about his wife or with Chris and Lisa. They were merely one of many triggers that could set him off.
raywest ★