My opinion, for all it is worth:
One of the many fabrics of the show itself in my opinion is innocence, and the absence of any such gore and more 'adulty' themed stuff is not only because it was a show originally aimed at younger ones, but also because it's nice to watch something that doesn't involve all of that, because it is horrible! I've never seen a brony talk about wanting gore, but it'll never happen anyway, because as I said, one of the main points of the show is to be fun and cute to watch because it has ponies! And that would be entirely contradicted if such gory content is added.
My opinion in summary: gore just doesn't belong on MLP. It's the wrong place for it. And as Little Riolu said, just go and watch an 18 rated movie if you want it