What choice does he have, in the end? To follow or die. To follow, or his most precious people die. To follow, or there is nothing left for him, not in the future, not from the past, not in the present. In the end, Draco has no choice.
He follows.
He obeys. He plots. He plans. He sets everything up. His plans are all drawn out. He sets them all into motion. These plans are well made. They should have all worked. Dumbledore should have dropped dead before Christmas came.
But he doesn't.
Because Draco, no matter how hard he tries, is not a killer. He is not a murderer. He doesn't have what it takes. Which is why he makes holes in the plan, tiny pinholes that make it seem like he really is trying, and the fact that it happened to fail was simply an act of fate or bad fortune.
But it's not.
He plots carefully, to make each move, each attempt realistic, that if he weren't deliberately causing them to fail in the most coincidental of ways, they would really be subtle enough to kill the old man.
He is cautious.
But the stress gets to him from time to time. It is not easy; he knows that one misstep, one mistake, one careless moment of indulgence may mean his life or death. He needs a release. He needs to get away.
So he cries.
He doesn't expect comfort. So when Myrtle talks to him for the first time, he is surprised. But he knows that he can't keep everything bottled up. So he makes her swear not to tell anyone, binds her with magic, and spills. Like a true Slytherin (because he is, although he's certainly acting a bit Hufflepuff-ish now), he makes her swear that if she tells, she wouldn't be able to stay in Hogwarts. And he is able to be absolutely honest for the first time in years.
It feels good.
But he doesn't expect that Harry Potter of all people would catch him in one of these emotional moments. He panics thinking maybe Potter has overheard. He is like a deer in headlights, grey eyes watery and wide. They stare at each other, and he glances at the door, hoping, looking for an escape.
There is silence.
If Potter hadn't heard, the attempts might have continued. He doesn't know exactly what would have come of it in the end. Looking back, he should have cast a silencing charm or a ward or a notice-me-not spell or something. But there is no use in thinking of what might have been now.
Because Potter had heard.