Well, time for the kid gloves to come off then. Severus took a sip of his drink, thinking.
The Dark Lord was dead and gone, his own purpose served - and he was dead, too, in the real existence. There was no reason to hide the truth from Lucius. Indeed, he worked in the bloody library; he knew or could find out whatever he wanted to know of the facts. Lucius was asking the details.
"I'm not sure what to tell you. She asked me to make it and I did. There were a great many things I couldn't do to help Draco, but keeping him from becoming a murderer was a powerful motivation.
"When I returned to the school and informed Dumbledore what I'd done, the Machiavellian bastard found a way to twist it in his favour. He'd got himself cursed earlier in the summer. I was able to slow its progress but I couldn't stop it. It was killing him. He decided that since he was dying, anyway, we should use his death to further his cause.
"I would kill him, when the time came, so Draco wouldn't have to do it. In so doing, I proved my loyalty to the Dark Lord beyond reproach. It was presumed, rightly so, that I would be considered the obvious Headmaster when the Dark Lord took over the school, and from that post I would be able to do what I could to protect the children there from whatever warfare was going on."
He had, too. Lacing all the food with Impotency potions to minimise the risk of rape, interfering with the Carrows' detentions whenever he could, pretending not to know of the Room of Requirement and the Guerrilla child-warriors within.
It had been difficult; loathed by those he'd once considered friends and colleagues - his last memory of Minerva McGonagall was her look of absolute disgust as she chased him from the school. But it hadn't been as difficult as living at the Malfoy Manor would have been, with Bellatrix and Fenrir and the Dark Lord all wandering about as though they owned the place. He remembered how Lucius had looked in the Manor those last few months. Wandless and helpless and little more than a servant in his own home, wan and wasted. Narcissa's strength had held the family together then.
"All of that came after the fact, though. The Unbreakable Vow was Narcissa's idea and I did it without hesitation. The rest happened afterward."