Please tell me how you determine legitimate and illegitimate. Do you actually think in your hubris that you have the ability to judge such a thing?  This is precisely why the UN was set up with out a "morality" standard, because who can really trust the motivations and the objectivity of the judges?
Syria was a sovereign nation with a rebellion.  The government in Damascus remains the legitimate government of Syria.  Once you start playing games with legitimacy you open the world up to all sorts of violent abuses.