Well, if it has to be metal, then Thorns' "Existence", "Underneath the universe" (partially part 1 and certainly part 2) & "Interface to god". I would also say the ending riff on Type O Negative's "Everything dies" (it's cut off on the single edit). May be their best riff and certainly the most "evil" and grand sounding IMO. Type O isn't really what I think of when I say "evil", but still... The riff in the "I'll do anything to make you cum" part in their "Be my druidess" too.
And when non-metal is considered, then John Barry's "Pawnee attack" from his "Dances with wolves" soundtrack. Also "Bogota 1984" from his soundtrack to "The specialist".
Maybe also Carl Orff's "Carmina Burana", however that's spelled.
There's not really much music I would call "evil". There's good, agressive, and so on, but not really much where I say "evil". "Pawnee attack" is the only one where I think "evil" is the only fitting word for the song.