That was the sort of purge that would be inevitable, of course. And it was the line he knew he couldn't cross without betraying Dumbledore a little too severely. "I'd be very pleased to help facilitate," he said. Was there any reason he couldn't simply let them do away with the muggleborns? The plot was in his hands, now; he didn't need to be limited by Dumbledore's scruples ... It was dangerous to start thinking this way.
"Good of Draco - I hadn't heard." That was another front he needed to start moving on. If they were both going to be at Hogwarts, it would be good to have his trust as much as possible.