It should be understood that the Dao Fruit is the manifestation of Cause and Effect. Shen Gongbao bears the cause and effect of the near-destruction of the Shangqing Daoist inheritance, so one can only imagine how plagued his Dao Fruit would be.
Some beings, the threat they pose while alive may be lesser than the Dao Fruit they condense after death.
Perhaps when Shen Gongbao was alive, his name as the Plague God was somewhat unearned, but after his death, as the catalyst for the destruction of the Shangqing Daoist inheritance, his cause and effect condensing into a Dao Fruit might truly solidify his name as the Plague God.
The Great Venerable, as a master of Yi Dao, even before the resurrection of the old fellows, was the first in Yi Dao, so he naturally understands the essence of Dao Fruit profoundly. Almost immediately after learning about Shen Gongbao's past, he determined that Shen Hou's Dao Fruit had issues, significant enough to affect himself.
