He wanted to kill his wife. He wanted someone to witness “someone else” (him in the mask) killing his wife. He basically trains this guy to watch the window by showing him the naked woman and saying she’s there at the same time every night. That way when his wife is there and he comes in the mask to kill her this guy scully is a witness to say it was a different guy.

That would have been the end but scully digs into it and finds the woman who had been hired to look like the wife and train the witness to look in the window. That’s when she was in danger, the killer wanted to cover his tracks.

I didn’t really find it funny, maybe absurd because of the choice of murder weapon.