I really disagree with the idea that "hua cheng hates everyone but Xie Lian" because it's more that we just hardly ever get to see him interact with someone he doesn't have personal beef with.
like the people in ghost city seem to genuinely like him. he's known to provide protection to ghosts being hunted by cultivators. he literally *ripped out his eye* to protect a group of random humans. he's not an evil demon king with a soft spot for this one guy. he is genuinely kind to people who aren't Xie Lian sometimes, especially people who exist outside heaven's sphere of "respectability" ie ghosts.
is he rude and abrasive? yes, but there is a difference between being nice and being kind. are his morals dubious? yes, but so is heaven; hua cheng just doesn't try to hide it. Hua Cheng is completely sincere and is transparent about all his actions both good and bad which contrasts him from the cover-ups and false politeness of heaven