But Son, what if I said this-
"My mother fed me chicken with beef."
Does it mean that she fed me chicken and beef or does it mean she used beef to feed me chicken (eg serving the chicken on a plate made of ground beef and using utensils made from beef jerky)? It could be either one, however silly the second option may be.
Now consider the Morgoth quote. When he "fed him with his own hand upon living flesh" perhaps Morgoth merely had his hand resting upon living flesh while he fed the beast and was actually feeding him apple pie. Of course I don't believe this to be the case and I certainly don't think that Morgoth was feeding the brute his own hand (sorry Twilight), but I don't think we can toss aside Twilight's idea simply by pointing out the word "with" in the text.