Letting an ambassador sit in an extremely dangerous city without sending more protection (which he wanted) is manslaughter
law is a lot more complicated than that, I don't know all the details behind the benghazi case (besides the fact that an extremely thorough republican investigation couldn't charge her) but some examples of things that would complicate things could be:
-if she was not directly involved in the decision
-was told/advised to do that in good faith by someone knowledgeable
-if the death was a result of extenuating circumstances that were not reasonable to account for (Maybe the particular part of the city he was in was generally safe, for example)
-if there was some sort of clerical error or error in how the protection detail did their job that significantly contributed towards the death
-Any combination of the above that makes her not criminally negligent (which is the requirement for criminally negligent homicide, AKA involuntary manslaughter)
It's got to be a lot more thorough than "this dude was put in a dangerous place and died because he didnt have good enough protection"