One idea/explanation has been offered. I'd like to offer another.
(Not because I think I'm right, necessarily - just to offer other potential viewpoints.)
Xanatos hired the Huntsclan, today.
This, of course, sounds horrendous - Xanatos paid off a group of murderous, anti-magical bigots to steal an item at a magical auction, regardless of who got hurt? Is he that selfish? That amoral? His assistant is a magical creature! His son is a magical creature? Why would he hire people to hurt magical creatures?
Well, he didn't . He hired them to scare magical creatures. They took up hurting all on their own, and Xanatos is not happy about it. See this passage from the update:
Owen clued us in on how his boss was feeling, here. The only reason he would do that would be if this were something that Xanatos was mistaken to try and hide - such as, say, his frustration that the people he hired are not listening to orders. See it from Xanay et al's perspective: it's unlikely any of this would be traced back to David, but it isn't impossible. And if anyone were going to notice, it'd be the person who had been hanging around nearby all day, and has repeatedly shown to be shockingly more insightful than his personality lets on. Xanatos might gamble for trying to keep hidden, but apparently Owen was like "Okay, we admit it, we did this, but we did not mean for things to go this far."
100 to 1 that whatever's in the box, it is either an immortality method or a first step to one. 10 to 1 that it's something Demona needs to do a plan, and Xanatos knows about the plan, and wants to stop her, but failed to grab it out from under her. But that's all besides the point.
Why hire the Hunstclan? Even telling them directly that they are not to be harming people, that's an extraordinary risk. Even assuming that there's some way to back out of the deal or make the money and/or power offered worthless - and of course there is, this is Xanatos we're talking about - having your name potentially attached to the Huntsclan? Potentially swelling their powerbase? What could possibly be worth that much?
Well, avoiding something that would damage even more people. In the cold calculations of Xanatos-style rationality, ten innocent lives is worth a lot less than the innocent lives of an entire populace.
Remember all the way back when Goofy initially made contact with Xanatos? He told Owen to send an auction invite to us, and upon Owen pointing out that Goofy, being a Toon, may have stumbled in on the Bazaar by luck alone, Xanatos responded:
"Naturally," ... "And if it is luck, it would be best to manage how Doctor Doofenschmirtz enters the magical community."
We're already flouting the Masquerade just by doing our own thing, and every King knows that. If we decide to take the same approach to the magical masquerade, once we know about it, the magical masquerade, worldwide, is going to be revealed. And people are not going to be kind to something that sudden, that strange, and that dangerous "suddenly" popping up.
After the auction we might still be able to hash out some kind of agreement with Xanatos about stuff he nabbed and we would like to at least have a peek at, like that cookbook.
What better way to convince us that the Magical Masquerade needs upholding than by providing an object lesson? And then, of course, narrating the object lesson. Just to really drive the point home.
It's the classic Xanatos Gambit. He hires the Hunstclan with the hiring being the primary goal, not the object they are hired to steal. That's just a bonus, a nice buttercream icing once the cake's been dealt with. The primary purpose, the cake, is us, thinking the way about the magical masquerade that we're "supposed" to.
...and if that, plus subtly shifting the Good Doctor to pick up the key for mutual benefits (at zero cost to David), just so happens to also result in Heinz Doofenschmirtz subconsciously adapting a position that, clearly, this Xanatos guy is pretty friendly, cares about innocent people, and really knows what he's talking about; it'd probably be a good idea to listen to him more often on future endeavors, well...
... so much the better. Even if his hired spook manages to catch on to the whole thing, he needs to uphold the Masquerade too. If he can keep Doof away from Xanatos's evil, evil ambitions, and still figure out a way to keep all the pillars standing, more power to him. Xanatos is willing to take a minor loss in order to keep Shego and Doofenschmirtz on his side.
...hmm? What's that, doctor?
"Nice going?" What do you mean, 'nice going'? Was that sarcasm? it sounded genuine, but...
...oh no.
Oh, no, I've said too much. I was trying to avoid being too subtle for him, but that speech on people being dumb, panicky animals was too overt.
Heinz Doofenschmirtz figured out everything that happened here today! Abort! ABORT! ABORT!
(in reality, we're just idiots)
