If there was a downside to knowing her husband as well as she did, it was the fact that he knew her just as well. That, of course, meant he knew how to get under her skin and provoke her. "I'd say it went rather well. His mission was taken care of, was it not? Dumbledore is no less dead. Draco is still alive and safer at school than anywhere else. What I did was not without flaws, but it kept him alive."
She'd kept it a secret from Lucius mostly because she wanted to make sure to keep him alive. What she did was no doubt punishable, as she'd been forbidden to speak of Draco's task to anyone. The only out she possibly had was that Severus and Bellatrix both already knew what Draco was meant to do. Nonetheless, she'd ignored the demands placed on Draco and if she included Lucius in the secret it might put him in danger as well. She wasn't sure what would happen should anyone else find out.