The Illuminati built weapons and Namor wiped out one alternate Earth, then jumped ship to form the new Cabal, who've been destroying every Earth that brushes up against the main Marvel Earth ever since. But I doubt Cap even knows about that one instance where Namor destroyed an Earth. He's after the Illuminati because he was a member, and when he raised on objection to them starting on the path toward world-destroying weapons, they erased his memory and booted him out. It's a personal vendetta and, worse, he's treating it as the absolute top priority of SHIELD and the Avengers, blinding himself to anything else going on. Yeah, it's certainly reasonable for Steve to want to find the Illuminati, find out what they were doing in secret, and mete out justice for their transgressions, but the way he's going about it in the "Time Runs Out" future shows he's way over the deep end.