Tbh even if it goes against international law and poses a problem of authority, I don't even see why they are trying so much to deny them that. What makes Ukraine and the west care so much for the Ukrainian flag over Crimea ?
What makes you say that's a coup d'état ?
Going above interim government and (appointed) head of parliament to call yourself president, enact reforms, call in new ministers prior to any form of re-election as agreed 24 hours prior with the EU and Russia? Then you got EU saying 'cool, we accept your new goverment' and suddenly everyone's acting surprised at the russians being not cool with this. That's going beyond a need for change and including the people straight into the territory of setting up a new reality for everyone to suck it up or bust, and apparently not everyone was cool with the euromaidan protests, and not everyone is cool with not getting a vote in the current government, hence people feeling left out and being a lot less accepting of current ukrainian politics.
Unless I have missed something, that's a rather biased point of view. To my limited understanding the deal specified that power had to be handed to parliament with a new transitory unity government to be formed, until anticipated elections happen in late 2014.
Parliament proceeded to broke the deal only by a time margin by making new elections actually happen right away rather than in late 2014, probably at the same time bypassing that planned "unity government". This is the real contentious point to me.
Nobody ever stepped up to the presidential throne and called oneself president... You may call it a con/show, but it still was a perfectly valid democratic election validated by the parliament, which had the power to do so (which was unwise imo considering the deals made with Russia).