I'm sorry to be confusing; I see cannibalism as orthogonal to death; one can amputate one's own leg and feed it to one's friends, or one can die of natural causes and permit others to consume the remains. In the grandparent, I wasn't considering dying being a part of the process of cannibalism.

As to dying for the purpose of being consumed, I don't think sane humans can consent to that, but other intelligences could, as long as they felt that the cost of dying was not high (i.e., they are confident that their goals will be accomplished regardless of their death). This is unlikely, but at least possible in my conception.