Beneath the Massacre isn't a -core band by any means and anyone who is an avid listener of "technical" music can tell you this. Musically, tech death is the antonym of deathcore and BtM's image is really the only reason why they might be erroneously classified as a -core band. I also went to a concert (the only concert I've ever been to) with Necrophagist and BtM and while Necrophagist predictably drew a larger following, BtM put on a better show and I think their music is more technical as well.
Back on topic, when I'm listening to metal, it's usally some kind of melodic, technical, progressive or symphonic death, black, or power metal. I've never been able to get into the older "classic" bands, but I don't like metalcore or deathcore that much either. I don't want to judge the older music too harshly, because most of it doesn't appeal to me because of the lower production quality, but -core music just bores the shit out of me. Personally, out of all the -core bands that I've heard, they're all basically just parodies of each other and all the songs, even by different bands, tend to all sound pretty much the same to me.
Also, metalcore and deathcore are both extremely easy to write. For those of you who have mentioned how formulaic they are, you're right and here are the formuli:
metalcore - i VI
deathcore - i vii°