Well, Severus understood that steely gaze well enough. Lucius was annoyed with him, and suddenly he suspected he knew why. Never having been a parent - indeed, having had no real family of any sort since he was fifteen years old, he couldn't quite comprehend the various jealousies and strife that seemed to surround them.
Lucius didn't like Severus's influence with Draco. Well, Severus hadn't liked Lucius's influence there, either. If not for Lucius's example and allowing Bellatrix so much access to Draco that summer before his sixth year, maybe Draco wouldn't have become a Death Eater.
"Well, she's Bellatrix. Someone has to keep an eye on her and I'll be bloody well damned if I'm going to leave her to Rabastan to turn her back into what she was before. I've always tried to protect my Slytherins ever since I was Head of House; I view her as sort of one my snakes."