There are two kinds of threatening patients: those who know what they are doing and are just miserable human beings, and those who are out of their minds for whatever reason and don't necessarily have control.
The latter kind need to be placed on a locked psych wing so those who deal with that daily can handle them (that's our job!
) The former kind need to understand that you *will not* be treated in that manner, and the fact that they are in a hospital bed does not give them special privileges to do or say things to you that they wouldn't do or say out on the street to a stranger.
Scenario: You walk into room to do your initial assessment on Mr. Patient and are greeted with, "What the (blank) do you want? Get the (blank) out of my room." You: (standing your ground)
"You've clearly mistaken me for someone you're angry at, because we've never met before and I'm very sure I've done nothing to you to deserve that kind of talk."
That particular line has shut down many an ornery patient in my experience.
If you get something along the lines of, "I don't need you checking on me," a good response is sometimes, "I'm very happy you're feeling well enough that you feel you no longer need our care. You should talk with your doctor about going home if that's the case. In the meantime, while you're here, I am obliged to monitor your condition."
Good luck - you'll find your own style. Always be safe, but understand that a lot of people are just blustering. Knowing that may happen and having a stock answer ready may help you defuse the situation.