What Is A Bohemian Rhapsody?

You all know the lyrics from Queen's Bohemian Rhapsody. The story is about a man confessing a murder to his mother, vainly pleading poverty in a trial and ending up resigned to his fate.

But questions remain: Who did he kill and why? And why does the judge talk funny? Who is Bismillah? And what about the Italian influences? Scaramouche, Galileo, and Rossini's opera The Barber of Seville?