Complex sentence: Cui Yan's true intention in writing those remarks was to mock Yang Xun's critics for being too eager to disparage Yang without making any careful consideration, but he was accused of displaying arrogance and defaming Cao Cao in his letter.
Split into simpler sentences: Cui Yan's true intention in writing those remarks was to mock Yang Xun's critics for being too eager to disparage Yang without making any careful consideration. However, Cui Yan was accused by some people for displaying arrogance and defaming Cao Cao in his letter.