You failed him by thinking his choice was made. It wasn't! There's still conflict in him. If he would turn from the Dark Side, that could shift the tide! This could be how we win!
You don't have to do this. I feel the conflict in you. It's tearing you apart. Ben, when we touched hands, I saw your future. Just the shape of it but solid and clear. You will not bow before Snoke. You'll turn. I'll help you.
I always begin them with geography and monster them until the have the world down cold. They may get lost in math; but, I'll be damned if they don't know where they are.
And a Jedi who saved him. Yes. Your father was the most hated man in the galaxy. He caused so much death and destruction for many years. But you saw there was conflict inside him. You believed that he wasn't gone. That he could be turned.