Baili Yen truly forgot, she and Baili Qi were both cruel people.
They were equally cruel to Ou Lan, someone who also shared a close blood relationship with them, so why blame the other for being cruel?
They couldn't be considered allies, only accomplices in crime. Allies have loyalty, but they didn't. They only had mutual use, and when interests conflicted, they would slaughter each other.
If asked during a crisis, would Baili Yen lethally attack Baili Qi to save her life? Of course, she would.
So, she really had no reason to blame Baili Qi for being cruel. She just didn't understand why, when there was no conflict of interest, he wanted to kill her now.
Baili Qi had not a trace of regret or pity, and even as he plunged the dagger into Baili Yen's chest, he felt immense pleasure.
Because of his fear of Zhan Moxiao and Xie Shichen, his spirit was stretched taut, like a string pulled to its limit, and killing brought him relief, thus his pleasure.
