If I have a weapon in my hand and yell "I will kill you" at someone, this is a threat.

If I you give me a teddy bear with a pun that I hate on it and I tell you "I will kill you" with a smile on my face, that's not a threat.

It depends on the context.

The first person would go to jail. The second person would not.

Analyzing the context of any situation is why we have a legal system with judges and juries.