I've always enjoyed this one. It's not a classic but it's fun and Falk and Shatner work well together on screen.
To me it seems that Ward Fowler actually has something of a breakdown during the events of the story as he starts to lose his grip on reality.
The only bit that slightly irks me in this story is the "Treasure to pill you" line. It's because it's so obviously written and I just couldn't imagine someone doing that naturally.
I agree. The more logical mixup would involve the first two letters of each word: "P" (if it's "Pleasure") or "T" (if it's "Treasure") and the "K" for kill. Following that logic, you would get, "It will be a kreasure to pill you." As I'm not aware of the existence of the word "kreasure", and neither were the writer's, apparently, they left it as we see it in the film.
Another thing I found interesting is that Fred Draper played the murderer in the most previous episode, "Last Salute to the Commodore" and had a significant role. His reward? He gets the shortest follow up appearance by any murderer in the Columbo series: one line - LOL!