Slave Qing was drowning his sorrows in the wine cup, complaining about the unfairness in life. He had paid a steep price - money, time, flattery, face - but in the end, gotten nothing.
Master Shangguan Fei did not hate him or like him; neither did he trust him nor suspect him either. In any case, he had become an easily-overlooked butler doing the most trivial things. From the looks of it, he would never achieve big things in life.
Hence, when the Tenth Young Master approached him, he leaped at the chance of a new master immediately. He was not a killer and did not have to stay loyal for life. Moreover, he needed money.
However, Slave Qing was not entitled to partake in the secrets but Shangguan Ru's request was also uncomplicated, precisely so that he could focus on the killers under the Ninth Young Master. This was to let their movements be as conspicuous as possible.