Horrorcore ain't just a Eastcoast thing
Insane Poetry, ADR Lavey, Mars, Lynch, X-Raided
Innovators of West Coast horrorcore, it's incorporated alot with gangsta rap, but that also helps it become more open to wider public
If anything I would say the Midwest has had the horrorcore scene on lock though
I understand what you are saying, and yes people from all over do "Horrorcore" if that's what you wanna call it, but in the beginning none of those artist you named other than maybe ADR and Mars really claimed the term horrorcore. Same with dudes from the midwest like Esham, ICP, and HOK. It wasn't until later that some of them came around, and used it to market their shit. Horrorcore the term is eastcoast and comes from eastcoast people, simple, point blank, period.
IMO it is all just underground hardcore rap. I have no problem with the label horrorcore, but I think it tries too hard to narrow down artists that have way different styles to one title. If there is one title for it all it's just "rap music".
And hey have any of you watched that Big Pun The Legacy documentary. One of his homies from the Terror Squad mentions Horrorcore and how Pun and them were big into it when they first started. It's pretty interesting.