In The Westing Game, who killed Sam Westing?

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.

In The Westing Game, who is Sam Westing pursuing?

I don't believe that Sam Westing is "out to get" any particular person. He sets up an elaborate "game" and/or murder mystery in order to help him find an heir worthy of his fortune. In that regard, he's "after" the person that can solve his game. That turns out to be Turtle Wexler, and Westing seems pleased that she won.

I suppose the other thing that Westing is after is the good of all of his heirs and the people that are involved with the game. He went to great lengths to pair them with the person that would bring out the best in each member. Additionally, Westing is a philanthropist, and he invests in people. He pays for the education of Turtle Wexler to become a lawyer, and he pays for the education of other people too. They in turn have an attitude of "pay it forward" and pay for the education of somebody else. Ford pays for Chris's education. Finally, the end of the book shows that each member of the group finds great success in the area that they dreamed of. Restaurants do well, an Olympian is made, happy weddings occur, and so on, and Westing is happy for each of those people.

Is Sam W. Westing actually dead in The Westing Game?

Sam W. Westing is not, in fact, dead. He has disguised himself as Sandy McSouthers and joined the other tenants in the game. It turns out that he actually had four different identities: Julian Eastman, Sam Westing, Barney Northrup, and Sandy McSouthers. He created the concept of the game so that he could bring all of the tenants together in an attempt to resolve some past issues and make them realize who they truly are and what they could and should do with their lives. The only person who knows that Sam isn't actually dead is Turtle Wexler, who became a good friend of Sam and visited him until he passed away of old age. Interestingly enough, Turtle is also the one who discovers his body, which turns out to be just a wax figure modeled after Sam, in the beginning of the novel.