Obi-Wan’s behavior at the end of Episode 3 is not all that Jedi like. I’m focusing on the end of his long light saber battle with Anakin/Darth Vader. Obi-Wan wins, they have one last discussion, and then Obi-wan walks away leaving Anakin to die in pain, and basically burn to death. That’s pretty cold, especially for a Jedi. Why not do one of these other options:

• Just finish him off. That would have been more merciful (I’m assuming Obi-Wan though Anakin would die soon after he left). There’s the obvious (Anakin can’t die here, or that messes up Episode 4, 5, and 6).
• Obi-wan could have used the Force to at least move Anakin away from the lava, or even take him back to the ship and get him medical help. Why just give up on his student and long time friend? This wouldn’t necessarily mess up the later episodes. The Emperor could have rescued Vader at some later point in time.