He shivered, hard. Stopped and stood there in the street with his head lowered, trying to imagine that. Trying not to imagine it. Realising how easily that could have been him. How nearly it had been.
Eventually, he raised his head, and looked at the Doctor with newly appreciative eyes, thumb rubbing gently over the back of the hand in his. "Suddenly, I'm rather glad I ran," he said candidly.
He was also thinking that what the Doctor was describing was uncomfortably close to what he'd been doing and was planning to do to Jack back on the Valiant. Not that that made him less inclined to follow it through when he got back. Doing something nasty to Jack was nothing like other being - especially Daleks - doing it to another version of himself.