Tort law was one of my favorite classes in law school, gruesome injury cases being more interesting than contractual disputes over international chicken shipments.
The usual rule of non-negligent mistake is that you lump it. I drive over a virtually-invisible oil patch and go into a skid and injure you. No one would've seen it, so the injuries are your problem.
Negligence is when I failed to see an otherwise-visible oil patch because I was adjusting the car radio instead of carefully watching the road, and this time I owe you. The classic Reasonable Person wouldn't have adjusted the radio except in absolutely safe conditions. The RP isn't superhuman, supersmart, or superskilled, but he or she doesn't make easily foreseeable mistakes.
Strict liability reverses the rule of non-negligence: if the harm was from something that even RP would not have avoided, the victim gets compensated. Same as the first case, I drive over a virtually-invisible oil patch and injure you, only that now, your injury results from the fact that I was transporting explosives that then exploded. Strict liability is considered a mostly-modern legal invention but there were earlier forms. Collapsing dams for watermill ponds were examples, and my favorite case was a pioneering, late 19th-Century balloonist who landed on a woman's vegetable patch. She hauled him into court for her veggies. He rightly pointed out that ballooning is brand new and no one knows how to land them - the judge said tough luck, if you do something abnormally risky like ballooning then you're strictly liable for any harm.
Negligence and strict liability seemed like separate concepts until Professor Grey pointed out that the Reasonable Person acts reasonably every time, but no actual human being does. It is unreasonable to expect someone to be reasonably prudent every time, but the law expects that, so a corner of strict liability is embedded in the law of negligence, presumably for the same societal reasons that we apply strict liability in other situations.
So this brings us to Benghazi - it's hard to figure out what the right wingers are screaming about, especially when their bizarre claims about coverups seem tangential to the real issue of inadequate security in the lead-up to the tragedy. I don't know if the inadequate security was a non-negligent mistake or negligence on someone's part, although I'd lean towards the latter. As far as the response once the attacks started and the hurt feelings of the people who believe they didn't get accurate information in the near-term aftermath, the first of those two things is hard to judge and the second isn't all that important.
But that still leaves the screw-up in the security preparations. Even if it's negligence that resulted in four deaths, I don't hold that as a major screw-up of the Obama Administration. They make thousands of security decision, and they will screw some of them up. Someone should pay for it somewhere in the chain of command (assuming it is negligence), but this is small potatoes - it would be unreasonable to go from this to concluding that the administration as a whole is negligent.
I wish the worst thing we could say about the Bush Administration was that they screwed up and four people died.
UPDATE: I need to do some additional research but I think Paul Ryan lied to the public on national television about a national security issue in the vice-presidential debate when he said there was virtually no US security in Libya compared to what we have at the Paris embassy, while knowing that CIA was nearby. He should get hit with this when he runs in 2016.