In that scene Superman practically begged Zod to relent, but Zod insisted. You could see the anguish in Superman's face as he knew what had to be done, but at the same time didn't want to do it. It went against everything he had been taught, everything he believed. In the end he saw no choice but to save that family that was about to be annihilated.
That's not murder.
I thought Superman was taught that life wasn't important, though? Isn't that what the scene with his father dying in the tornado is about? That a secret is much more valuable than human life, since life is only temporary anyways? But secrets and protecting yourself above all else, that's forever?
He basically murdered his father through inaction anyways.