How to assassinate Dumbledore had, at some point, ceased to be Draco Malfoy's concern alone.
Especially after Snape had staked himself on it as well, Voldemort had practically given tacit approval to their operation, allowing Draco Malfoy to draw on the strength of other Death Eaters to eliminate his lifelong nemesis—Albus Dumbledore.
As for punishing the Malfoy family, it was simply not worth mentioning compared with killing Dumbledore.
Because if even Snape failed, Voldemort would have no chance of eliminating Dumbledore in the short term and turning the tide against the Ministry of Magic.
That would mean an even worse situation for him and the Death Eaters.
No one could guarantee that the Death Eaters would remain loyal to him without breaking after enduring such an intense campaign of suppression.
Only a little over half a year had passed since Scrimgeour became Minister for Magic, yet the original Death Eaters had suffered grievous losses under the Ministry of Magic's crackdown.
Even though the Death Eaters had already gone underground, their situation remained dire. After all, their enemies were not merely the Ministry of Magic and the Order of the Phoenix. There was also an even more insidious and ruthless Master of Prophecy targeting them in the shadows. Some Dark Wizards who had originally sided with them had already fled after realizing the situation was turning against them.
Though Voldemort did not care about the departure of those fence-sitters, nor did he care much about the casualties among the other Death Eaters, he had to keep an eye on the power he had built. Once he lost the Death Eaters' support, he would be left utterly alone. When facing the entire vast British wizarding world, he could only hide himself carefully like a rat in the sewers.
That was also the