I thought it was Obi Wan’s pride and hubris as said in the novelization of Return of the Jedi.
He thought he could teach Anakin just as well as Yoda had taught him.
Then we get to the prequels and Obi Wan’s Master isn’t even Yoda it is Qui Gon.
I also thought it was a slow seduction to evil. Not a turn on a dime. Not murder younglings or lose Padme.