Do I need music theory for metal?
Short answer
No, not to start. You don't need to read notation. But knowing a few things, like what a power chord is, the natural minor scale, and which notes change the mood over a riff, makes writing and soloing much faster.
What theory actually does for you
Plenty of great metal was written entirely by ear. Theory is a shortcut for that ear work. Instead of stumbling onto a dark sound by accident, you know "that's the flat 2, that's why it sounds evil", and you can find it again in any key.
The theory worth learning first
- Power chords: root plus fifth, and why they work under high gain. See what is a power chord?
- The minor pentatonic and natural minor scales: most metal lives here.
- Two color notes: the raised 7th (harmonic minor) and the flat 2 (Phrygian).
- Chord tones: which notes sound finished over a riff. See targeting.
All of it is covered with tab in metal scales for guitar.