The legal Slippery Slope regarding Americans joining ISIS

Here's the problem: If someone is fighting in another country as a soldier of ISIS it is fairly straightforward that if the U.S. blows this person up with a drone it is legally acceptable even if this is a U.S. citizen.

However, what if someone advocates verbally their alliance or support of ISIS? Does the U.S. have the right to kill that person without trial in the U.S. ?

I have really greatly simplified this problem.

But, here is where I'm really going.

What happens when a Sunni Muslim child expresses interest in ISIS in the U.S. ?

Can they be imprisoned or killed for toying with these ideas here in the U.S. ?

Where do we draw the line as Americans?

Here is the legal slippery slope that will affect the right to live for all Americans.

So, let's get it right so Americans that are just not wise or good at making decisions because they are too young yet don't get killed for only thinking about something before they actually do something where the U.S. military, police or even private armed citizens feel they have no choice but to end that person's life.