I always thought that this episode had one of the weaker motives for murder. Tony may have been the "weak link" in the crime, but if there was no murder, there wouldn't have been much of a crime. Tony and Jarvis would only have been guilty only of breaking the trust before they were supposed to. As executor, I think Jarvis may have had the right to do that legally. Assuming he didn't, it's still not much of a crime to worry about.
Jarvis would have had half of the trust (about $1 million U.S.) if he didn't kill Tony. After killing Tony (and destroying a valuable automobile), Jarvis had twice that amount, but it was in cash! How can he spend it with police watching him?
It's a great episode, but the motive seems to have been an afterthought.