The Force has a fate of it's own, interpreted by the matter of perspective of The Dark Side and the Jedi Order. Who is to say who or how or what the proper course of The Force is? Self agency is prevalent throughout the lore for those with force ability, so I guess we can just settle upon the how on our own accord. Is it fate? Is it choice? Mixture of both?
Did Obi Wan really stand a chance against the will of The Force to prevent Anakin from turning? I really don't know, but the poor guy sure took it personally in a galaxy far, far away and for a long, long time.