How did our clever and cautious arch-Slytherin let himself get manipulated into doing such a foolishly Gryffindorish thing as to make an Unbreakable Vow with Draco’s mother—and without so much as requiring her first to spell out her terms?

Had he learned nothing at all from his previous blank-check dealings with Tom and Albus?

So. Let’s remember Luna’s maxim (in my fanfic “Protean Charm”) for figuring out the machinations of the smartest

JKR is very, very middle class: she hates aristocrates (they're meanies who hunt foxes and think they're better than her!) and her idol is Jessica Mitford, The Debutante Who Ran Away From Home And Joined The Communist Party (she even named her daughter Jessica in honor of her). Her loathing of the Upper Class shows in her books, with 'purebloodism' (whatever that may be) as being the Root of All Evil, and all her 'baddies' being of 'rich, snobbish, pureblood Ancient Houses'. As a good little Middle Class girl, she naturally abhorrs things like foxhunting, which she sees as the ultimate example of why the Upper Class is evil, and so we have Bellatrix who kills foxes with glee and that Black great aunt who wanted to legalize the hunting of muggles.

At the last moment that Harry was conscious, it was Harry who was the aggressor deliberately causing his foe unbearable agony and grievous bodily harm—Quirrell was merely struggling to escape.

Of course, it could indeed be the case that Twinkles realized that “the effort involved nearly killed” Harry as well as Harry’s victim, and intervened to save the boy’s life, not the man’s.