Complex sentence: Lü Bu behaved arrogantly in front of Yuan Shao because he felt that he had done the Yuans a favour by slaying Dong Zhuo, and he belittled Yuan's followers and treated them with contempt.
Split into simpler sentences: Lü Bu behaved arrogantly in front of Yuan Shao because he perceived that he had done the Yuans a favour by slaying Dong Zhuo. He belittled Yuan's followers and treated them with contempt.