Arizona's top prosecutor investigating Trump's comments about Cheney as possible death threat

Will no one rid me of this meddlesome priest?
-Henry II of England preceding the death/murder of Thomas Becket, the Archbishop of Canterbury, in 1170.

This is exactly what Trump is doing

Here's the quote in context:

“She’s a radical war hawk. Let’s put her with a rifle standing there with nine barrels shooting at her, OK? Let’s see how she feels about it, you know, when the guns are trained on her face. You know, they’re all war hawks when they’re sitting in Washington in a nice building, saying ‘Oh gee, let’s send 10,000 troops right into the mouth of the enemy.’ But she's a stupid person, and I used to have, I have meetings, with a lot of people, and she always wanted to go to war with people.

Can they arrest him? He's not the president right now.