Neighbor with Delusional Disorder (?), Please HELP!

We have a neighbor who screams obscenities to himself alone in his house, and who recently freaked out and accused another neighbor of beating on his door in the middle of the night, stealing his mail, and pushing him. He became beligerent and extremely verbally aggressive, but with no reason because the accused neighbor was asleep with his fiancee at the time. He now thinks I'm involved and called me names because I tried to intervene during the altercation. Please help, I don't know if this is Delusional Disorder but I know he's already Bipolar and this is scaring me. I don't want to feel unsafe in my house or neighborhood! Any advice? Thanks!!!