cannibal corpse is a death metal band, absolutely, no question. They have never, ever been close to a grind band. The aesthetic for grindcore is entirely different. Napalm death was a grindcore band, they are now a sort of mish mash of styles, which some like and others don't. You may be incorrectly using the term metalcore to mean influenced by hardcore, by which standard you have a point, at least in napalm death, since they were always part punk. Metalcore the genre however is a rather specific form of music which uses the "new school" (read: crappy) approaches to both metal(shadows fall, lamb of god, god forbid) and hardcore (hatebreed et al.) to form bands like unearth, killswitch engage and the like. It's metal with all of it's "metalness" stripped out of it. Just loud distorted guitars, and shouting and whatnot, but with an image that's just chic enough for today's discerning hipsters.