Lucius really had no evidence that Rodolphus intended to use an Unforgiveable, but his intuition told him it was a possibility, especially judging by the serious expression on Lestrange's face, and the excitement which abounded on Bellatrix's - and he could not afford to take chances. All their futures were at stake here, but it was himself he was most concerned about. He had a future with Narcissa to consider, and his path lay with Lord Voldemort. He didn't wish that to change because of anything these two might choose to do.
The teacher in question had managed to regain her feet, and hastened off. No one else was about. He held the wand out to Rodolphus with a sneer. "In future, I recommend you find better ways of entertaining one another," he said before he turned on his heel and left them.
"Bastard!" he muttered, "thinks he can tell us what to do because of his father, and because he thinks he is better than we are." He was breathing heavily, his wand still in his hand, his emotions completely on the surface now, not neatly compartmentalized as he usually kept them. He needed an outlet, and he needed it now.
He reached for Bellatrix without thinking and pulled her to him, pushing their lips together, kissing her with all the passion contained in his tempestuous nature, thoughts of Lucius and revenge fleeing his mind as it filled with the warmth of her body instead, pressed against his.