He wonders whether he could have done something to prevent this.
It's a question he'll undoubtedly labour over for the remainder of his existence, whether that ends thirty seconds from now with the swipe of Anakin's lightsaber or uncounted lonely decades into the future.
He has caused this somehow. He must have. He can't bring himself to truly blame Anakin, and there's simply no one else left.
The person he loves most has destroyed everything important, including himself, and Obi-Wan should have stopped him.
He has become accustomed to heavy burdens, but this one is still almost impossible to bear.