The Buddhist establishment slandered Xiao Tiezhu for a simple reason.
Years ago, when the venerable Miao Shan attempted a blood sacrifice to avert calamity, he ended up losing the duel, perishing and extinguishing his Dao, effectively severing the Buddhist lineage.
Initially, Buddhist monks wrote books that smeared his name, and later on, ordinary monks, not knowing the details, added their own interpretations when annotating scriptures, truly coming to believe that Xiao Tiezhu was the enemy of Buddhism.
Two hundred years have passed since Xiao Tiezhu's death.
After ten generations of laypeople under the influence of Buddhist scriptures, countless believers consider him the World-Ending Devil King.
The Daoist sects cared a bit more about face, merely branding Xiao Tiezhu as a heretic and excluding him from the ranks of the revered sages, all because he had hindered too many cultivators in his lifetime.