Albus Dumbledore is normally competent to defeat Voldemort, Bellatrix Lestrange, and any three other Death Eaters in arguably single combat without even really breaking a sweat. He's also normally approximately impossible to assassinate. Unfortunately, he put a little too much faith in Alastor Moody, who normally more than merits it, but unfortunately had himself gotten very unlucky nine months ago when he took one (1) thirty-second break from maximum paranoia to drink one (1) beer and immediately got got by Crouch Jr. (a prodigiously talented duellist) and Pettigrew (an illegal Animagus who knows a way above-average amount about Moody's personal habits), a combination of things even he had not actually expected to encounter even though he was normally very good at being prepared for arbitrary bullshit. 

So Alastor Moody, one of the very few people Dumbledore trusts to sit behind his wand arm, isn't himself. 

And Albus Dumbledore, looking away, his habitual baseline defenses aimed everywhere else, is not ready for a Killing Curse to the back of the head, just as Voldemort and a dozen Death Eaters teleport past the wards and directly into wand range of a crowd of undefended civilians and students. Something like half the Order (or what would have been the second version of it, if they'd ever started having the second war instead of lost it in its opening volley), unprepared, is scattered through the audience, desperately spinning up enormous shields to try to protect the kids, but there's too much attack surface. 

Even Minerva McGonagall cannot win that fight when it's handed to her without warning. 

"Most of the students have been evacuated," explains Remus, back in the present, tightly, "but Hogwarts is lost and so is everyone else who might have been interested in defending it and-- look, I know you don't like me but you're all we've got left and your family is famous for pulling absolute bullshit out of your back pockets when the chips are down, so I'm asking. What have you got?" 

(Who, her? Pulling absolute bullshit out of her back pocket? Lies, heresy, and fiction. She had nothing whatsoever to do with any of those things that happened during various wars that would definitely for sure have been Statute violations. She is very respectful of the Statute.) 

"Right. Well. I feel as though I should be offended but in fact I did recently acquire about 97% of an absolutely unhinged research project on changing Time," which Yelena would absolutely have finished if she hadn't finally managed to murder her, her mom had mailed her a congratulatory bottle of really good firewhiskey about it, "and if I remember your Runes specialty correctly, I suspect you might be able to put together the other 3% and use it without blowing up the bloody planet." She sighs. "Able and willing, however, differ. How do you propose to convince me that you will not use it to blow up the planet." 

"You have what--" (no, stupid question) "-- how-- why--" (nope those are not helpful either) "--of course I won't blow up the planet! I live here!" 

"He says, with all his friends in Heaven and all the monsters here." 

Wow he does not like feeling that seen. Fuck. 

"... point taken." 

"Got a better answer, or am I thanking you for the information and--" kidnapping and Imperiusing Rookwood, probably, oof, that's going to suck massively-- "trying something else?" 

"Right. I'm not going to insult your intelligence by claiming to have Lily's ethics. My real answer is that I'm a coward and I would actually rather live the rest of my life depressed and alone than find out where I go if I die in the process of committing that many murders." 

... okay that tracks. And is depressing as all get out, but that's rather the Marauders for you, isn't it, either blindingly privileged glittering perfection or breathtaking tragedy, there's never any middle ground. 

"Could not be me," she says dryly, "but yeah, all right. Come in, let me show you the notes. Mind the third stair, it will try to kill you."