Superman did kill in the Sliver Age.
Just only "monsters". There were a few stories were he fought monsters that were even more powerful than him and he was forced to kill them.
But never, ever humanoids.
DC Comics Presents Superman and Supergirl. Kal and Kara were doing their best to kill Mongul by casuing War World to overload the brain synapases in Mongul's brain.
At the end of the story
For the Man Who Has Everything, Superman was going to drop Mongul in a black hole.
Jimmy Olsen and the Magic Totem, Superman tricked a Kryptonian criminal into killing himself.
The Superman from the Superman cartoon killed the Parasite by allowing the Parasite to drain almost all of his energy and the Parasite blew up from being overloaded.
The Superman from the Superfriends cartoon was faced with a hard chioice. He changed time by tossing dampening rods into the core of the sun Rao when he accidnetly went through a time-warp and Krypton didn't blow up. He went back to Earth to find it devestated, because he wasn't there to protect it, and he returned and removed the dampening rods. Krypton blew up and Earth was saved. One way or the other, he was going to kill billions of people.
Try to explain the last two on the Superman doesn't kill stance.