KrazyHades said:
If eating a chicken that was once a human is cannibalism, then eating a human that was once a chicken is perfectly fine.
Correct. In fact, if eating a chicken that was once human is cannibalism, then killing a human that was once a chicken is perfectly fine* provided that you know that it is a chicken at the time you do it.
And one other important point...
Since the creature takes on the ability scores of what he's becoming, I'd say that it's NOT cannibalism, because with the loss of intelligence comes the loss of humanity.
...provided that the chicken hasn't acquired whatever traits make something human. That could be intelligence or it could be something else. It's not at all clear that humanity must be defined by the presence of intelligence. I think alot of people would argue that a human is still human absence any real or detectable intelligence. Even if your chicken is currently a bird brain, it could still be intrinsicly human if there is something intrinsic to being human other than either the body or the mind. For a much more interesting take on that theme than I can manage, read virtually anything by Robert Silverburg.
I think it is a reasonable ruling that intelligence = sentience = humanity. But I don't think it is a necessary one. There are plenty of other takes on the question, especially if you begin with the assumption that people really have spirits, which is generally the default assumption of fantasy.
*I say fine, but there is a caveat here that any act undertaken for a cruel and sadistic purpose is wrong sufficiently for that reason regardless of whether it would otherwise be right and proper. If you are turning chickens into people just to increase the pleasure you get from killing them, this is wrong. It's not murder (though its probably practice for murder), but its still very wrong.