In The Sting Lonnegan had a professional hit-woman, Loretta, work at the diner that Hooker frequently visited so that she could kill him. I'm guessing that with the bet ready to go the next day, Lonnegan doesn't need Hooker anymore and can have him killed off, saving himself some money.
But Loretta ends up getting killed first by a protector setup by Gondorff.
Shouldn't Lonnegan have been surprised when he arrived at the gaming room and sees Hooker was still alive?