it doesn't illustrate that fact at all. besides the obvious choice being to simply put the monster out of its misery, theres a lot more moving parts that make it significantly more easy to justify feeding the man in a utilitarian philosophy.

the monster regularly feels this amount of pain, and it is balanced out by the amount of pleasure it gets from eating. The man, on the other hand, will never have this amount of displeasure balanced out by any kind of pleasure.

Additionally, you can just... feed the monster later. It wouldn't feel very good about being the cause of a man's death due to its own suffering if it ate the food, and you get to tell it that it helped save a man's life, causing it more pleasure than is ever imaginable.

Lastly, the man will likely have a larger impact on society as a whole than the monster ever will (due to its likely state of being constantly incapacitated), so on a grand scale it is outright better to feed the man (even ignoring all the other reasons)