Goldfinger question?

In the movie version, Operation Grand Slam was never about "the heist."  It was a terrorist plot from the beginning to destabilize the U.S. economy by irradiating its gold reserves with a "dirty bomb."

The "thugs" were just used by Goldfinger to gather the equipment and weapons needed.  This is a reference to the American Mafia's influence in transportation, industry and labor.  If you listen carefully to the uncut theater version, there is even a quick reference to how one of the mob bosses helped to get a large number of men (soldiers who he called "labor")  "across the border."

Killing them was just a classic example of the movie villain trope "cleaning up loose ends"...getting rid of everyone else involved with the plot to avoid discovery later.