How to assassinate Dumbledore had, at some point, ceased to be Draco Malfoy’s burden alone.
Especially after Snape threw his own fate into the balance, Voldemort tacitly approved their operation, allowing Draco Malfoy to enlist the help of other Death Eaters to rid himself of his lifelong nemesis: Albus Dumbledore.
As for punishing the Malfoy family, compared to killing Dumbledore, it was simply not worth mentioning.
Chapter 1162