Horrorcore is like Hardcore rap/hip-hop that is heavily influenced by horror and gore. It sometimes is also called Gore Rap kinda like how Necro calls his music Death Rap. It has some metal influences specifically from goregrind but it is not metal. Check out MC Bushpig Scatbutcher one of my favorite horrorcore artists.
https://scatbutcher.bandcamp.com/album/wank-of-death