In the abstract, this is a good argument for international law (IL). IL would be a good thing. But it does not necessarily follow that any violation of IL should be condemned, just as a cop breaking a law to protect an innocent person's life might be excused, depending on the particulars.
In this case, atrocities are happening in real time and can't wait for the machinery of IL to prevent them. Even worse, if we take the UN as the embodiment of IL, members of the Security Council have an inherent interest in vetoing certain acts in defense of innocent life. For instance, China and Russia have an interest in squelching any support for the self-determination of oppressed nationalities, in light of the situations of Tibet and Chechnya. On some matters, the U.S. is no saint either. Trying to improve this state of affairs is a worthy undertaking.
Meanwhile there will be situations that can't and should not wait for IL. I agree that usually might will not make for right in the absence of effective IL. In the specific case of Kosova, a serious effort to rescue Kosvars that entailed serious resistance from Serbia would still be right.