Well, as always, who decides the leader is illegitimate? Are the Saudis illegitimate, according the the rubric we put on Maduro?

The UN deliberately has no mechanism for this because it's a talking shop intended to help avoid war by providing a talking venue. That's the whole idea, they're not the world police, there is no such thing. They're a forum.

I'm absolutely not defending any given dictator but history shows that every attempt to remove a dictator "for the greater good" is usually 1) selfishly motivated and 2) backfires horribly.