A metal song? You could write about a number of things. Here are a few of my favorite topics for metal songs:
1. Kickin' someone's ass
2. War, death, destruction, etc.
3. Wizards, warlocks, dragons and other fantasy bull****
4. Rockin' out, head-bangin'
5. Gettin' wasted
6. Gettin' laid
7. Anger and aggression in general
8. Politics and current events