If 4200 dead were sufficient to warrant US intervention, then we would have troops in almost every nation on the planet.
In the run-up to war, the kind of numbers that were asserted by the Clinton administration were in the hundreds of thousands.
Clearly, the war on Serbia was a complete fabrication.
You don't appear to be able to follow the conversation - the war was fought to end Serbian repression of Kosovar Albanians and prevent refugees from destabilizing the Southern Balkans.
The "100,000 dead" issue arose from a statement to the effect that there were over 100,000 Albanian men unaccounted for inside Kosovo, and given Serb atrocities in Bosnia, there was reason to fear for their health.
Does the lack of finding WMD's in Iraq or the hyperbole surrounding the issue prior to our invasion invalidate our actions there?
No.
Same - same with Kosovo.