I created a a hostile NPC who attacks anyone who gets near him using the Aggro radius. However, when he attacks, the guards only watch the fight instead of protecting the victim.

How can I make the guards attack the hostile once he starts combat?

Set the guards and everyone who isn't an aggro guy to be allies, and set their behaviour to Helps friends and allies.

I don't want to do that, because these are the normal city guards I'm talking about.

Have a script that on the actor then that sets his faction as enemy to the Guard faction.