Nope. That was easy. If some American is plotting to blow up other Americans and you know it, then arrest him and charge him. You don't get to just blow them up.
How many times a year to you hear about a police sniper shooting a hostage taker because they believe other people are in imminent danger? No arrest. No charges. Just a quick kill because there was no other way to prevent them from killing someone. Think of the Predator as a sniper on steroids and again, if there is no other way to prevent the taking of even more innocent lives, why not use it?