How should we deal with a possibly insane neighbor?

So one of my friends (let's call him friend A) lives with his sister in an apartment. There are two neighbors, a drunk and a fucking maniac. When we (me, friend A and another friend (friend B)) first found out something was up with the crazy neighbor was this one day, we had been sitting in the apartment for a couple of hours when we decided to leave. The entire time we had been in the living room we had heard sounds of him violently smashing on things in his apartment, slamming doors etc.

When we where on hour way out, friend A and B were fighting a little bit (you know, that kinda teasing fighting) in the hallway and i was putting my shoes on. All of the sudden the maniac neighbor rips his door open. We all stop and look at him, and he looks at us. For about 2-3 seconds he gives us a look of "what the fuck do you think you are doing?!" then says in an angry voice "smartasses" then he closes the door by slamming it shut as hard as he can. We are all a bit freaked out by this but we don't thing much more of it.

Some days later me and friend A where on our way to the apartment. As we open the door to the apartment, the crazy neighbor rips his door open and looks at us for a second. He asks for friend A's sister's boyfriend but he wasn't home. So we told him. Before friend A is even done saying that one sentence explaining that the boyfriend isn't home he says "yeah, if there's not quiet soon, i'm gonna come up and stab somebody" and then he slams the door shut with full force.

Later that day, friend B comes over and we are sitting upstairs. When B comes up he tells us that the neighbor just threatened to beat him. For no reason what so ever.

I weren't there For the episode that happened today but friend B told me about it. The neighbor apparently didn't say anything this time when they were entering, he just lifted a fist up ready to punch friend A in the face, then slamming the door shut as always.

We are fucking terrified by this guy. Any advice on how to deal with this?

possibly useful info:

He was extremely nice when friend A and his sister (and her boyfriend) first moved in, telling them that if his music was too loud they should just tell him and he would turn it down. I heard him say this and it was in the nicest nice-guy voice.

It's a couple months since they moved in.

He's apparently been hospitalized at a psychiatic hospital.

TLDR: Friends neighbor is crazy, threatened us with stabbing and beating even though we've done NOTHING to provoke him.

Sorry for the spelling, english is my second language.