I suppose the blue checkmark is supposed to prove that the person that account is claiming to be actually said the content of the tweet themselves, or at least endorse it. It would make sense that after their death, that can no longer be accurately said, however, the account lives on, and someone else might have access to it and use it for their own messaging, as with Cain.

But removing the checkmark from the account entirely seems like a dumb way to combat this. IMO the checkmark should stay on all tweets that were made from the time the person was verified until their death, and only not be shown for any tweets that came after.

But I agree that this may not be intentional malfeasance, just a bad solution to a relatively new problem.