Who may have murdered Sam Westing in The Westing Game?

No one murdered Sam Westing because he isn’t dead, but all of the residents of Sunset Towers are suspects. 

The Sunset Towers apartment building was specifically designed for a select group of people who had a special connection to Sam Westing.  Technically, they are all suspects at one time or another.  This is why they are chosen to live there and to participate in the Westing Game.  However, it turns out that Westing has been alive and watching them under various aliases all along. 

Westing was a paper products tycoon who made a lot of money and then disappeared. No one has seen him in years, and until the heirs moved into Sunset Towers, the Westing mansion was empty.  You can see the Westing mansion from the apartment building, and one day there was smoke coming from the chimney. 

Westing specifies in his will that he was murdered. 

I, Samuel W. Westing, hereby swear that I did not die of natural causes. My life was taken from me—by one of you!

The police are helpless. The culprit is far too cunning to be apprehended for this dastardly deed. (Ch. 6) 

This pronouncement is obviously intended to pit the heirs against each other and make them suspicious of one another. The heirs are not sure what the purpose of gathering them together is.  They realize they all have a connection to Sam Westing in some way.  When the will is read, they are paired off to participate in a game to win his inheritance.  However, most of them still think that he was murdered. 

“Murder? Does that mean Westing was murdered?” Sandy asked the heir on his left.

Crow turned away in silence.

“Does that mean murder?” he asked the heir on his right.

“Murder? Of course it means murder. Sam Westing was murdered,” Mr. Hoo replied. … (Ch. 6) 

A lot of them have motive.  Crow is Westing's ex-wife, and she definitely has bad blood with him because they divorced after their daughter died the night before her wedding.  Mr. Hoo sued Westing, claiming he stole his invention.  The others could easily just want to kill him for his money.  Someone has been stealing, and bombs keep going off.

It is actually the youngest player in the game who figures out that Sam Westing has been there all along.  He has masqueraded as Barney Northrup, Sandy McSouthers, and Julian R. Eastman.  This way, he could keep an eye on his game.  Turtle discovers the truth, and wins the game.  They become good friends.  Sam Westing later dies of natural causes.