What are some things to think about when you're trying to solo over something evil sounding? Metallica, AIC, Black Sabbath ect.
diminished arpeggios
augmented anything

job for a cowboy, some evil solos

'08 Ibanez S5470 Prestige
Gibson SG Special - discontinued teal finish
Mesa Boogie Triple Rectifier
4x12 Mesa Oversized Slanted Cab
iSP Decimator
If you try to solo with something evil, try using diminished whole-half or a half-whole scale, also a locrian mode as a really dark sound in it.

But if ur trying to solo over metallica or black sabbath, use the natural minor scale of the key and if you want to use something special use the harmonic minor scale if it sticks to the key.
Normal you achieve the sound you want with phrasing. It's a bit hard to describe what I'm thinkng with phrasing. Phrasing is definantly what defines a solo.

After that its probably whats being played underneath, so if you have a dark riff or progression playing you wont need to do much for your solo to sound dark.

The next important thing is note choice. I like the diminished arpeggio suggestion, but doing diminished arpeggios too much can start to sound a bit unstable, so use it in moderation. What's coming in my mind, at the moment, is some slow bends, like maybe a flat fifth bent until it sounds like a perfect fifth.
[U]        | |                     [/U]
[U]        |/     .-.              [/U]
[U]       /|_     `-’       |      [/U]
[U]      //| \      |       |      [/U]
[U]     | \|_ |     |     .-|      [/U]
      *-|-*    (_)     `-’
^Right on. Theres so much emphasis on "what scale" but really its more about phrasing. Just keep improvising and play slowly and feel the notes your playing. Don't worry about speed just worry about playing nice phrases.