He probably wasn’t expecting Han to try and confront him? Someone you haven’t spoken to in years might provoke that reaction.
I got the impression that the moment Han stepped onto the bridge he knew what was going to happen.
Ben/Ren is so conflicted that Han is willing to be killed just to take the pain away from his son.
He doesn’t take the weapon away or point it away from himself he lets it sit there. Han the cynic has made his peace with the universe and is doing whatever he can no matter how weird and twisted to make his son feel better. Which for me, reading it that way was a very sophisticated character interaction. Even if plot wise it’s Kenobi letting Vader kill him all over again.