Another way to add that extra evil twist to a song is dissonant chords (for lack of a better, or 'proper' name) like the ones that Six Feet Under uses.
The following is the second riff in Remains of You:
|---------------------------------------------------------|
|---------------------------------------------------------|
|---------------------------------------------------------|
|-----------------5-----------------------------6---------|
|--2--------------3--------------5--------------4---------|
|--0--------------3--------------3--------------4---------|
If I remember right the second and fourth chords are a 5th-Root-5th construction, which sounds evil as hell if used sparingly. Below is a standard C Power Chord (Root-5th) and a Dissonant C Power Chord (5th-Root-5th).
