I've taken a pretty strong "intervention never justified' stance, but I admittedly know little about the balkans, and if intervention ever were justified it would be in the case of genocide. I've heard Chomsky and Parenti called genocide deniers and justify the bombing because Serbia was doing that genocide. It sounds like every anti-communist smear ever created, but I know there weren't really any "good guys" here
NATO doesn't do interventions, it does sociopathic bombing campaigns designed to destroy its enemy states.
The question of whether an "intervention" war justified is premised on the propaganda that NATO is some kind of peacekeeping defensive force.