If people from the Divine Halls found out that Ning Que was there, he would definitely be killed.
Ye Hongyu thought about the life and death vow because she felt that Ning Que was either courageous, or very stupid to come in front of her at the Seat of Black Heavenly Jade. This was akin to placing his life in her hands.
She sounded extremely bitter. The life and death vow very aptly described the situation Ning Que was facing and the purpose of his appearance on the Peach Mountain.
What is love? People live for it and die for it. It was normally the cause for suicidal craziness. Ning Que did not come to the Peach Mountain for her, nor for the imprisoned Chen Pipi. He was clearly here solely for the person in the Divine Hall of Light.
What a stupid choice it was to walk to his own demise because of love.