Gazerbeam, for instance, isn't merely dead, he's really most sincerely dead.

No question.

And you'd think that most of those Supers would've sounded the alarm rather shortly after escaping Syndrome's trap, if they managed it. Sure, go to ground for a while, but given Nomanisan Island as the killing ground, you'd think it'd be pretty obvious that you weren't the ONLY candidate. A few might not care, but most of them would react the same way Mr. Incredible did at the idea of some psycho killing off Supers.

This is a problem even without faked deaths. I don't remember which version of the Evil Killing Machine was finally sent out against Mr. Incredible, but all the previous versions had gone against another super and lost. In no instance, as far as we know (and barring some sort of government cover-up) was any sort of alarm sounded.