She stands in the middle of her own living room, arguing with her husband over his mistress she had just found out about. "Why, Lucius? Why can't you just stay here?!" her anguished cry tells him more than he needed to know.
He notices her cry. It pains him to hear it, for even though he has his mistress, yes, he loves her more than he loves himself. But he is so blinded by his love for both wife and mistress that he begins to shout. "Because I love her!" as soon as the words leave his mouth he regrets not saying, "and you, too."
She's pulling out her wand now, giving him a soft, sad smile. She sees what's going on with him. She can see the love in his eyes, and misconstrues it. She's turning the wand on herself, and before she can think, she knows what she wants to say.
Lucius can barely get out the strangled, "No!" that leaves his lips just as she whispers the curse unto herself. He realizes what she used. Narcissa was dead before she hit the floor. He cursed himself to hell for causing his wife to kill herself.
Hiding halfway in the doorway, Sarah realizes just how hopeless her case really was to try and get Lucius to ever stop loving his wife, even though she was dead now, and quickly mutters the appropriate incantation, falling to the floor herself, wand rolling away from her on the hardwood floor, rolling just out of reach of her motionless fingertips.
For the first time in his life, Lucius Malfoy feels totally helpless. He's a lost cause, causing two women to kill themselves in one day. Tears he hasn't shed since he was 13 fall now, and he curls in upon himself, sobbing as hard as he possibly can. Then in comes Draco, who figures that now that all the shouting was over, so was the fight.
What he sees shocks him. His mother and father's mistress are both lying on the floor to the right and left of him, eyes glassy and wands right near, self-inflicted. As for his father, who he had always known to be strong and subtle, he is lying on the floor between the two women, sobbing and curled into the fetal position.
Draco, showing nothing to betray his thoughts on the whole scene, goes over and just tries to pry his father from the hands of his dead lovers, trying desperately to get him up off the floor. There's a meeting tonight, and he won't let a dead wife and mistress make his father miss it. Draco knows the consequences for mistakes.
Draco knows that his mind has been royally fucked up by the Death Eater organization, but he also knows that his father will never be the same. He knows he should take up Harry Potter's offer on protection, but he also realizes the boy will fall to the greater powers the Lord was amassing every day. Once the war is over and Lucius has proven himself weak by crying, Draco realizes that he will be made head Death Eater, which is too good a chance to pass up. Once the boy falls, everyone's lives will change. All impure blood will be flushed out and terminated. Purebloods will reign sovereign once again, just like the old days. He's going to take the Granger mudblood for his own, watch her squirm under his wand as he shows her a new meaning to pain. He had hoped his mother and father would be alive to see the day he defeated the Dark Lord and instituted his clever plan to rule the world as he saw fit, but it seemed as though it wouldn't work out that way. He'd hoped it wouldn't come to this. He'd hoped he would be able to escape without murdering, take over the world and still be secure that when he looked back at the day, unlike the Lord, he could say he had killed no one. But it just wasn't meant to be. Draco knows his mind is twisted and warped, but he realizes it will just have to stay like that. He knows that sometimes you have to sacrifice one person, or in this case three, for a greater, better cause.
And you know what? He's right on every one of those counts. He rises, pausing to crouch and look in his father's living eyes for the last time, tell him he loves him so Lucius would at least die happy, kiss him goodbye for the last time, and raised his wand. Lucius was still a moment after.
Draco puts his wand away carefully, dragging the bodies outside, grateful for the high hedges Narcissa had been so insistent on, carefully digging three holes, and placing the two women on either side, Narcissa right and Sarah left, just as they had been when they died, and Lucius in the middle. Draco takes special care to keep him in the same position he had died in, comfortable in the knowledge that he could look back on this day and say that he had killed not one person. No, he had not indeed. The two women had turned their own wands against themselves, and Lucius would have basically become dead anyway, and Draco considered it an act of mercy that he even thought his father worthy enough to send on to the next life.
Later on he reclined on the couch with his homework for the summer, rubbing his forearm absent-mindedly and waiting for the Lord to call him into service. He would institute phase one of his plan to take over wizarding Britain tomorrow. It was only a pity his parents weren't alive to see this. Oh well, he supposed quietly, done is done.