"How did everyone fall for him despite of the obvious red flags?"
An IRL recently asked me this and I think it's good to share my response here, too.
Since the very beginning William Gold sold us, his fan base, on the idea that Wilbur Soot was his digital persona. A fictional character / alter ego he acted through. In SootHouse, in the ARG, the comedy songs, and in normal streams. Wil was meant as a parody of incel culture. So anything super alarm worthy was dismissed as in character.
This became a major problem for Wil when the DSMP exploded in popularity. As suddenly thousands of new eyes were on the first song of the Boyfriend Trilogy and his old work. Wil rushed to remove the most offensive and criticized works of his. He had to morph the Wil character from a creepy freak into a more tolerable one; landing on a charming, self aware, famous egomaniac.
He made this switch and tried to paint more separation between 'himself', Wilbur Soot the character, and DSMP!Wilbur. But often would use it as a shield for criticism. When he said rude / did things people assumed it was a joke.
That's why he got away with this for so long. He openly joked about biting Niki all the time and we, the fans, didn't bat an eye. It's not the audience's fault for having literal years of content telling us not to take him seriously.