> I guess I understand why they broke the agreement (keeping Yanukovich until December is, well, quite contrary to what protestors seek), but what I really fail to understand is why they ever agreed to that.

The "they" you are referring to are not one group - the first "they" are the protesters, while the second "they" are the opposition leaders. The first "they" broke the agreement that the second "they" made because they never agreed to it.