He leaves that sorta thing to Nick or Bucky, if they ever even come up.
He has an alternate plan to destroying another world... the infinity gems. That might not work, but I think it's fair that he wants to try that first. And they all seemed to agree with him that it's at least worth a shot.
If he had NO other option and he still refused to destroy the other world, I might agree with you. But they're not at that point yet.
Getting AWAY from the standard bitching about AvX and trying to paint Cap as a genocidal warlord...
There was the time when he had to chose between killing Baron Blood and having the vampire regain his full strength.
Then there was the time Cap had to chose between using a gun to take down a soldier of ULTIMATUM or allow him to continue to fire into a crowd of innocent civilians.