In vast majority of states there is a stand-your-ground law in place, which means you're explicitly allowed to use deadly force if you reasonably believe that you're defending yourself from a violent crime, and in minority there is a duty to retreat while outside of your home or workplace, which means that you still are allowed to use deadly force, but you have to try to retreat first, and only use force if you are reasonably sure that you can't run away.
Kidnapping by a group of people in plain clothes in an unmarked van is explicitly stated as an example of a case where use of deadly force is permitted by law.

I think that would count as self defence.

you're almost there

I'm pretty sure self defence doesn't apply against law enforcement, even if you don't know they are law enforcement.