No, it wasn't.
They were powerless, and unable to do anything.
So what if they threatened him? They had no way of actually carrying out those threats. It was already established in the PU Superboy story and then reiterated in Superman #22 that gold kryptonite meant a permanent loss of power. He had even planned to kill them before they threatened him, as he had kept the green kryptonite on location.
It was an execution, plain and simple.