Although knowing Chiang Qing was the one who commissioned Mi Shaoan to kill Xiao Nianxin's parents, and despite Mi Shaoan's testimony, the actual reason remains unknown since Chiang Qing is still lying on a hospital bed unable to speak, not to mention confrontations, so we can only wait.
Old Master Cheng heard the 'story' from Cheng Su and is inclined towards her explanation: personal vendetta and ancient grievances, so he struck mercilessly.
Old Master Cheng states that time should not be wasted on unnecessary people. As for Chiang Qing, she's right under our noses; as soon as she can speak, the answer will be known.
But if she never speaks, then the answer won't matter; what matters is that she didn't achieve her wish, Xiao Nianxin is still alive, and she will eventually face punishment.
