I can see the problem with this as presented. As noted elsewhere, in the Golden Age there was the excuse of patriotic rallying to that sort of image.
In retrospect, Cap NOT killing Hitler without a seconds hesitation when given an opportunity, IS an issue. We're not talking Batman not killing the Joker here, both are fictional. Here we have a representation of a real, unspeakable monster, you don't use a sleeper hold on him, you snap his neck. And yet at the same time, we know Cap never did do that. It's just unsatisfying on a number of level (in the sense that this could ever be a satisfying issue)