Exactly, and if they ARE Silver Age Kryptonians, then Supes had a very good reason for doing what he did here. Think about it. First off, how many times did Superman get his powers taken away in the Silver Age? How many times did he find a way to get them back? Dozens - hundreds - maybe even thousands! I don't buy this 'Gold Kryptonite takes your powers away for good' stuff - this is not taking into account the existence of Red Kryptonite, which has COMPLETELY unpredictable effects. It could give you an ant's head, or turn you into a giant, or flip-flop your personality - or, quite possibly, re-power depowered Kryptonians. And I do believe there is a chunk of the stuff in Superboy's lab, along with the gold and the green varieties. All Zod and company have to do here is expose themselves to Red K over and over again until eventually it has the effect of giving them back their powers. Then it is quite possible that they COULD find a way to Superman's world - and that could turn nasty. Second, Silver Age Kryptonians were several orders of magnitude more powerful than the current version. It was generally made clear that Silver Age was basically more powerful than ANYONE, with the possible exception of the Spectre and other Kryptonians. If I remember correctly (and I should state in advance that I'm just going on hearsay here, since I haven't read any comics with him in it), Superboy-Prime was a Silver Age Kryptonian for all intents and purposes, and look at all the damage HE did. Imagine three adult versions of him - three ANGRY adult versions of him - three angry adult versions of him with a grudge. And Supes doesn't have Gold K to take their powers away again - or even if he does, it might not work on them, since they're from another dimension. They'd kick his ass - and after that, they'd kick everyone else's ass - and after that, they'd destroy the world. So basically, they may have been helpless at the time, but if he'd let them live, there was at least a 50-50 chance that they'd get repowered and come to destroy his world. Supes is a highly principled guy, and I'm not surprised that making such a decision wrenched him around a bit, but really, what choice did he have?