To be fair, Yun Shishi treated Mu Xi well. To her, Mu Xi was not just an assistant but a friend. She had even invited the girl to be her bridesmaid at her wedding.
This meant that Mu Xi was someone of great significance to Yun Shishi, she was like family.
But Mu Xi betrayed her. This was what she could not get past! She trusted her wholeheartedly and did not doubt the assistant's words at all, believing fully whatever she said!
She rarely trusted someone this much, however Mu Xi was Qin Zhou's subordinate. Hence Mu Xi's deeds were truly heartbreaking.
When Qin Zhou found out about this, he was furious. He even thought of punishing Mu Xi severely, but he was busy taking care of Yun Shishi at that point, so he could not spare the time.