Who is the murder in the book the westing game?

There is no murderer, actually.

Sam Westing was the same man as Barney Northrup, Sandy Southers, and Julian R Eastman.

He faked his own death, so in effect, he "murdered" himself -- or at least his popular persona.

Sam Westing was not murdered. HE was the "murderer" - he "killed" his own fake identities, all four of them. He was a lover of games and he wanted to play one final game with his family and friends as a whole. First he killed immigrant Windy Windkloppel. Next millionaire Sam Westing

(himself). And then doorman Sandy McSouthers and lastly, his final identity, Julian R. Eastman.