It is still a threat under the definition, but because he said "kindergarten" instead of naming a specific kindergarten I am saying it is protected speech under the 1st Amendment.

There are exceptions, but generally, it is not criminal to discuss committing a crime, in order to constitute a crime there usually needs to be a substantial step in furtherance of committing the crime.

Yes exactly.

It's the difference between "I'm so angry I could kill someone right now!" and "I'm so angry I could kill so-and-so neighbor right now!"

There's also a huge difference between screaming the latter to unrelated people and screaming it to the so-and-so neighbor.

Not exactly. He didn't say he "could" shoot up a kindergarten, he said he was going to. Is there a difference between naming a specific kindergarten? Absolutely. But that doesn't mean only one of them is a threat.

In my example, both use the 'could.'

I really don't think that someone should be charged with 'making terroristic threats' - nor any kind of threats - with just a vague statement of intent.