It's not clear that you can really cast it as a successful effort to save lives either, given that it's plunged Libya into violent, sectarian civil war.
Would the humanitarian cost be less if NATO hadn't intervened? There was already a civil war in progress in Libya, after all. NATO didn't and hasn't gotten involved in Syria (with the exception of a limited air strike this year in response to the Syrian government's use of chemical weapons on civilians, a strike of limited strategic value) and the situation there is one of the worst humanitarian crises today.
I think people are too quick to pass judgment on US or NATO intervention in matters and aren't acknowledging the extreme complexity involved in making the right decision. When dealing with some of these international conflicts, sometimes it's less a matter of making the right choice but rather making the least awful choice, and it's often impossible to determine the true consequences of decisions in advance.
