General:
* Aggressive
* Hostile
* Antagonistic
* Combative
* Warlike
* Pugnacious
* Contentious
* Quarrelsome
* Argumentative
* Bellicose
*
More specific:
* Threatening: If you want to emphasize the potential for violence.
* Volatile: If you want to suggest a quick temper or tendency to erupt in anger.
* Confrontational: If you want to emphasize a direct and challenging approach.
* Incendiary: If you want to suggest the potential for violence and causing trouble.
For a person:
* Warrior: If you want to emphasize the person's fighting spirit.
* Fighter: If you want to emphasize their willingness to fight.
* Hothead: If you want to suggest impulsiveness and anger.
The best synonym will depend on the specific situation and the nuance you want to convey.