Anyone holding more than a few bitcoins technically has a price on their head also. 

I think the media stories about tontine murders are just for theatre. If we follow that logic, once a Last Will and Testament  is written, everyone in your family becomes a potential murderer. 

The solution anyway is to use cryptography to protect the identity of the members of the Tontines and even better, when the number of members in the pool falls below, let's say 100, then fix the distributions.

This means that way before there's any practical incentive for foul play to kick in, you have already even removed the incentive