Terrible
Say you're Albus Dumbledore. You are, for the second time in your life, fighting in a great war of magic. You are the leader of the Order of the Phoenix, noble, brave wizards who have vowed to stay the tide against the Death Eaters.
And, not to mince words, you're losing.
You've heard the whispers, that you're not the wizard you used to be, that you're just too old. You suspect there'd be a quiet coup, if there was anyone who thought they could do better. And maybe they're right. Living long enough to see a former student rise to such power in such a terrible way—you are not a parent, will never be a parent, but you imagine you have felt some echo of the pain a parent might feel, in seeing their child they feared for for so long go so far astray. It certainly aged you, and perhaps you are now too old to do what must be done.
And yet... you are starting to suspect another factor at play. Tom-Voldemort-never had anything but disdain for quidditch, yet he seems to have a Seeker's luck, constantly snatching victory at the last possible moment, escaping unscathed from routs that seemed certain to kill. Perhaps your old pupil is not merely lucky, but has done something, something terrible, to cast away Death's gaze. And if that's the case...
Say you're Albus Dumbledore. On the one hand, you have this suspicion. On the other, you have a prophecy, a real hope, in a time when hope seemed to have been in hiding. Tom already knew, of course, Severus told you that much, half mad over guilt for what his former master would certainly do to Lily, if he caught them. And now hope truly was in hiding, until Harry could grow to fight Voldemort on even footing, for, prophecy or no prophecy, how could an infant possibly vanquish the Dark Lord?
The question eats at you, as if it were... a riddle, though you don't much care for the irony. An academic question, at first, but the answer comes to you late one night. A dying mother's love, according to fable, had a power to it, and perhaps something that terrible would be enough to triumph over another something terrible. But it's unthinkable. Your mind shies away from it immediately, repulsed by its own musing.
But how many lives would Tom take in the interceding years? How many other families would be ripped apart? A decade, two decades hence, what would Britain look like if Voldemort was still in power? Wouldn't it be worse, to stand aside and let that happen when there was another way? You continue to think the unthinkable, watch the Order fall to pieces around you, and you make a choice.
You know where they are, it was you who suggested the place, though you felt a twinge of guilt already, sending them back to your home. But you need to find who else they trusted with the secret. You can't leak it yourself—too much a risk that Tom would link it back to you, and smell a rat. And that phrase, that exact phrase, answers another riddle, reminding you of four boys who thought they were being terribly clever, hiding animagi right under their teachers' noses. Of course James would have entrusted it to Sirius, and of course Sirius, who by large was still that boy who thought he was terribly clever, would have given it to little Peter, in turn.
From there, it's easy, so easy that you think you may be guided by the prophecy itself (a pretty lie, but it doesn't take away the guilt). You find Peter, bolted up in a shack in Diagon Alley, and fix him with an Imperius Curse, and tamper his memory to make it seem like his own idea. It doesn't take more than the smallest nudge—the pressure was literally killing poor Peter, a responsibility greater than he could bear. Any release was preferable, and you, relentless, brought him to it.
And so Tom chases after them, as you knew he would, and so he kills the parents first, as you knew he would, and so he seems to parish, as you so desperately hoped he would. And Peter seemingly dies at Black's hands, and you don't look closely, or speak in Sirius' defense, and it's a terrible thing to do to an innocent man, but not so terrible when stacked against what's already been done. And what's been gained. For while you fear it's not Voldemort's end, you have time, you all have time, to regroup and rebuild, to marshal forces and make plans.
The boy grows up, and you love him. You love him as you loved Lily and James and Sirius and Peter and Tom. You love him as you shape him, mold him, turn him into what you need him to be, what you need him to sacrifice. It is all out of love, you tell yourself. You have to. Because if someone did what you did out of anything but love, if they did it out only of desperation, out of despair, out of fear-
Well, wouldn't that just be the most terrible thing of all?