* Khaddafi ordered his soldiers to shoot non-violent protesters * There was (as a result) an active rebellion against him that could use some support

There was no invasion by western forces, and I think the air support was mostly to stop Khaddafi from massacring his own people. I don't think western forces ever assisted in an attack; it was fairly limited and mostly denied Khaddafi the advantage of his air force.

A thumb on the scale to tip the balance between Khaddafi and the rebels, basically.

And in fact, Syria is not so different, although the involvement is on a much larger scale. It's still mostly various rebel factions that do the actual fighting. Iraq, by contrast, was a full scale invasion, and one based on a lie.

I'm not saying that makes Libya and Syria completely justified, but it's clearly a different case than Iraq, and much easier to justify, if you accept the legitimacy of the rebels.