Mr. Monk and the Class Reunion - S5-E6
Question: In this episode, Monk discovers that Kyle intends to murder his wife Dianne and then make it look like suicide. Why would he do that? I've watched the whole episode and still can't figure out the reason.
Answer: When Dianne was in college, she had attempted to commit suicide and even wrote a suicide note. Dianne saved the note this whole time and Kyle had found it. If Kyle killed Dianne and made it look like a suicide, the suicide note would be authentic (rather than having to try and forge a suicide note that could be proven to be fake). This is why Kyle arranged for specific activities to occur that seemed odd to Monk, the dog having the same name, the missed pass that broke the glass, etc. These were all mentioned in the original suicide note.
How did Mr Monk know about the suicide note?
Because Dianne told them about it, Stottlemeyer went to talk to her when he found out she sent a wreath for Kathrine (the woman killed at the beginning), and Monk was there at that time. Dianne said Kathrine saved her life when she was depressed in college, and she had even written a suicide note.
Answer: It speaks for your character that you cannot relate to his reasons. But he wants her dead, because he is having an affair and since all their money is coming from her father and his high paying job is also not his gain but another grant of her father he would very likely loose everything if he divorced her for another woman. And as the other answer mentions already he sees his opportunity to get away with it by re-enacting the circumstances as described in her 25-year old suicide note.
Answer: You can hear Kyle talk to his "mistress" on the phone when he is at the ice cooler in the hotel, thereby making it clear he has an "extramarital" affair, thereby outing his motive to get rid of the wife who brought the money into the marriage...plus her life insurance, supposedly.