to get a metally "evil" sound, you might try a lot of flat 5's flat 3's and flat 2's,


here is a flat 5, --0------6---

flat 3, ---0------3---

flat 2 -----0---1---

and im not sure what the chord is called that i use but i like this one the best,
(low 3 strings)


-------7---------
-------3---------
-------5---------


and of course the dissonant chord which is used a ton in metal, especially band like oh, sleeper, and the dillinger escape plan
(low 2 strings)

----------3---------
----------7---------