I believe Voldemort only marks those that pass specific tests that he administers. I don't think he's mark anyone that couldn't perform all three Unforgiveable Curses. I have no doubt in my mind that Lucius has killed someone, if not many people, before. He was ready to do it in CoS just after Harry freed Dobby. He was halfway through uttering it before Dobby threw him back. If he's that willing to do it in public, even more so in a school, there's no doubt he'd do it anywhere. As for Draco, I think Voldemort branded him (if he truely did) just to play with his food before he ate it. He knows that Draco doesn't have it in him to kill anyone. I think that's something he can sense in a person. He's just having Draco do those tasks because he know he can't do them and he's punishing Lucius for failing him so many times. Knowing that Draco can't accomplish what he's set out to do, he'd have to kill him.