All the evil that Anakin did was out of love. He needed a new power to save Padmé, the only way to gain that power was through the Dark Side so he had to turn, so we don't know for sure that he turned on free will. The relationship between Anakin and Obi-Wan was pretty okay. But he still is a Jedi. Friends or not. Obi-Wan is an enemy and could prevent the gaining of this new power to save Padmé. You see that in the lines when Padmé and Anakin arrive on Mustafar.