“You’re next” is not a threat.

Contextually, it 100% is a threat. She preceded “You people are next” with the same message left behind by Mangione at the scene of the murder, a recent event that everybody involved in the conversation was aware of. Sure, if that statement existed in a vacuum, it’d be an open-and-shut case, but it was part of a larger conversation.

Also:

she reportedly admitted to using those words during the call, telling detectives that “healthcare companies played games and deserved karma from the world because they are evil.”

So yeah… she’s gonna need a really good lawyer.