I think it is a good idea to control population and add food supply, but this is only possible if we ignore cannibalism. Or maybe we can export the meat for starving Africa ? And I think it is much cost efficient than breeding livestocks and more efficient on land-use
To use death-sentenced criminals as a food source means that more would have to be killed to meet demand. More being killed means that more people will be sentenced to death. More death sentences means that, eventually, corruption will happen, and people who would have been sentenced to a mere 10 years or so get their whole lives dragged from them, because the judge was paid off under the table by the death row inmate.
The criminal was already killed. I do not think it is morally acceptable to punish him in the afterlife as well. The person already paid for their crimes with their life. I think we owe it to the criminal, and especially to their family, to give them a proper burial.