I see each attack having way different circumstances and the only commonality was Americans being killed.
9/11: an unforeseen attack (to the populace, not the govt.) where people from unfriendly countries attacked Americans on American soil. Out of the blue, so to speak, and a massive but limited attack on a couple of selected sites, not an invasion.
Libya: a country in the hotbed of the ME where war, tyranny and uprisings are SOP, and where our embassy on their soil was attacked as part of a larger series of attacks throughout that area. Attacks related to US killings of Libyans by drones.
The way I read it (and this might be totally wrong) is that the RW critics are looking at it simply as the death of Americans, and the LW critics see it as the death of Americans within the broader turmoil in the region, some of which was exacerbated by the US.