I think Paul Waldman has a point about why the right is relentlessly focused on the Benghazi killings, but I don't think this is the whole story:
So what's going on here? I can sum it up in two words: scandal envy. Republicans are indescribably frustrated by the fact that Barack Obama, whom they regard as both illegitimate and corrupt, went through an entire term without a major scandal. They tried with "Fast and Furious," but that turned out to be small potatoes. They tried with Solyndra, but that didn't produce the criminality they hoped for either. Obama even managed to dole out three-quarters of a trillion dollars in stimulus money without any graft or double-dealing to be found. Nixon had Watergate, Reagan had Iran-Contra, Clinton had Lewinsky, and Barack Obama has gotten off scott-free. This is making them absolutely livid, and they're going to keep trying to gin up a scandal, even if there's no there there. Benghazi may not be an actual scandal, but it's all they have handy.I'd say that, in addition, right-wingers are desperate for the facts to fit their theories about Obama. They think Obama hates America and wants America's enemies to win; they think he, as a dirty lefty, wants the agents of our foreign policy, both civilian and military, to come to a bad end.
So, with regard to Benghazi, they think he did something sinister because doing something sinister to harm our military and diplomatic personnel is what the Obama of their caricature would inevitably have done by now. They've been waiting for him to undermine America -- they think it's impossible that he could fail to undermine America, given the opportunity -- so this must be what they've been waiting for.
Sure, they think they have evidence of his evildoing, but his true nature (even though it's all in their heads) is really all the evidence they need: he hates America, so of course something evil is afoot.