Dead Skin Mask is about the serial killer Ed Geins, who if i'm not mistaken was inspiration for the Texas Chainsaw Massacre, and after learning that it took on a whole new light for me. I agree about the Motorhead song though, just the title even haha.
also, there was this European ambient-horror group, can't remember their name at the moment... but they have some of the most terriying, atmospheric ambient music I have ever heard. It literally sounds like what you would hear in Hell.
Definately Dead Skin Mask, first time I heard it was live at Donington '92(first time I heard Slayer, actually), I thought it was a good song but when I went out and bought Seasons and learnt the lyrics it creeped me out but it did start my interest in serial killers, especially Ed Gein, and I think that's where my love of horror films came from too.
Plus it's a killer riff to play on the guitar too!