How can you prove that the person sending a death threat was specifically motivated by a specific, broad act of speech? This is an impossible, unenforceable pre

How can you prove that the person sending a death threat was specifically motivated by a specific, broad act of speech? This is an impossible, unenforceable premise. It has no basis in law either in theory or practice. Example lines of inquiry: How does one identify the “first mover”? How does one identify the integrity of the “calls”? What if the calls are a single caller represented separately? What if the calls are falsely represented by motive?