I don't know that I've ever thought of the 'weak link' motive, but it is obvious how much Uncle Jarvis despises his nephew's perceived lack of backbone. I think I would lean toward Jarvis killing Tony for money and distaste. He really dislikes Tony and his wife. While getting the $300,000 is great, there must be some sort of perverse satisfaction in ridding himself of his annoying nephew once and for all.

As for Ken Franklin, I have always thought he wanted to kill Jim out of anger and as a sort of revenge. Columbo noted Franklin "never really showing any true emotion for a man you worked with for 15 years". At the end, when Columbo shows him the story outline which Franklin used to kill Ferris, Franklin temporarily relishes the attention, as it touches his immense ego (oh, look, even Lt. Columbo recognizes the brilliance of my work). However, it also sets off his feverish jealousy, as he indignantly remarks, "I must have shared that with him five years ago. Who ever thought that IDIOT would write it down."

This is a good topic to explore.