Keep in mind that Fate/Stay Night takes a lot of liberties with the source material, but for what it's worth, yes, Excalibur does explain why Arthur fights Mordred, why there is rebellion/disillusionment amongst his knights, and stuff like that.
Sorry to hear you didn't like Unlimited Blade Works, I personally rather enjoyed it. In terms of plot is suffers from being too short (in the visual novel, Unlimited Blade Works was the longest route, longer than Fate), but I thought the action scenes were pretty awesome.