Question: Why do Snoke's guards attack Kylo Ren and Rey after Kylo kills Snoke? They no longer need to obey him, and he is past protecting.
Answer:Kylo Ren has betrayed the First Order. They were loyal to the First Order. You are assuming they only did so out of fear of Snoke rather than out of loyalty.
Answer:Kylo killing Snoke is no different than any leader being assassinated. If the U.S. President is assassinated, the Secret Service will come after the assailant (s) even though the president is past protecting.
Answer: Kylo Ren has betrayed the First Order. They were loyal to the First Order. You are assuming they only did so out of fear of Snoke rather than out of loyalty.