In order to achieve Dao Fruit, he had to eliminate the Abominable Lord!
However, the Abominable Lord was a parasite of the world's Heavenly Dao, and doing so would naturally be a bit tricky.
Fang Yuan was very doubtful whether the Abominable Lord could erode the entire Heavenly Dao. It was basically impossible. In fact, the chances of him being assimilated were higher.
As a former Netherheaven Demon God who had reached the peak, the Abominable Lord probably knew this as well. Otherwise, he would not have focused on the punishment aspect of the Heavenly Dao and arranged a deathtrap for him.