She had to save them.
She was anxious to leave the Demon Realm and return to the human world to save her parents' souls.
However, Feng Qingtian never showed up.
After some thought, she looked for Gu Yunjing. She couldn't wait for Feng Qingtian any longer.
With their strength, the Demon King wouldn't necessarily be able to stop them from leaving the Demon Realm.
The Demon King wasn't too wary of her either. He probably felt that she really wanted Feng Qingtian to die, so he trusted her.
If this were the past, she might've agreed like before to kill Feng Qingtian.
But after what happened last time, she wouldn't do that again.
In fact, she still hated Feng Qingtian for hiding so much from her when they met in this life. He had used the same tricks again to win her heart and make her fall for his warmth and love.
He clearly knew that for her, this love was hopeless.