If he weren't a Jedi, Obi-Wan would have raised his shoulders in a simple shrug. A shrug to say that he couldn't answer that question. Could anyone answer it? That was hard to say.
"He couldn't accept loss. He wanted to cheat death. He was in pain. He was suffering. He was afraid. Every Jedi will tell you, these feelings can only lead to the Dark Side."
Anakin had a choice. All Jedi had a choice. Obi-Wan had faced a similar decision when he was a young man, fighting to find his place in the universe; his place in the Force. Anakin chose not to accept defeat. He wanted to change the fundamental truth in the universe.
Eventually, we all lose the ones we love.
"I don't mean to say that anything Darth Vader did was acceptable or justified. It wasn't. Anakin made bad decisions. Bad choices. He did many horrible things. Unimaginably horrible things. But--"
And then it was as though Obi-Wan were thinking out loud. His attention was no longer on the princess. His gaze seemed to be somewhere in his mind, not out in the real world.
"I had a choice. I could have refused to train him..."