I think a fairy deal has power based on the implied integrity of the fairies to honor them. It's not so much Dust himself making a deal but Dust making a deal based on his bond as a fairy. By knowingly breaking the deal the fairy queen hasn't made Dust personally culpable but impugned the integrity of fairies as a whole to honor their bonds.
Yeah, I get all that. As I've said, my problem is with the mechanics of how the deal was broken. Usually in fantasy stories, this show included, the nullification of a normally unbreakable magical contract or bond requires some major, dangerous ritual with incredibly rare ingredients or a blood sacrifice or a deadly quest or something of the sort. It's usually portrayed as very hard or costly to achieve, which is why it doesn't happen all the time. Just slicing her hand and drawing an unreadable symbol on the wall seems pretty banal for such a thing.
I mean, if it's that easy for a fairy to break their deal, then it goes against the idea of fairy deals being ironclad in the first place. Sure, the principle is that fairies choose not to break their deals because they know their power depends on keeping them, but they can't all march in lockstep all the time. Any random fairy could've potentially made a bad decision or gotten drunk or gotten greedy and decided to break a minor deal without realizing the consequences, and then that random act of stupidity would've destroyed all fairy deals everywhere forever. So it seems like the only way the integrity of fairy deals could've lasted this long was if breaking them was very hard, not just something any fairy could do at any time by writing a bit of blood grafitti. Perhaps the intent was that only the queen had the power to do it, but then they should've said so. They should've done more to sell the idea that this is an unprecedented, extraordinary act.
I mean, if breaking a fairy deal is something the fairies would never even contemplate, then why do they even have a symbol that allows breaking them just by writing it? It seems contradictory. This needed more explanation.