First time Hens aren't all that bright and don't always have the Mothering instincts required past going broody... But it's hard to say why this is going on, you never really know with Guineas, especially if these are young first time Mommas.
Here's a few more thoughts:
-Maybe she could have felt something was wrong with the clutch, so she's following instinct and nature is taking it's course (sad but it happens).-
-Maybe the brooding Hen is so frustrated from the lack of privacy from the other Hens that she just pecks at anything that moves, including freshly hatched keets (like I said, not all that bright).
-Maybe the other Hens that are not brooding the nest could have gotten aggressive and killed the keets as soon as they emerged thinking they were a threat to the nest.
Do a head count on all the Hens, if one is missing she may have left with a few keets. If that's the case I'd find her/them, take the keets and raise them in a brooder. You could use a hardware cloth divider wall in the brooder to separate the injured keet until it recovers enough to be with the rest (if there are any). In the mean time I'd put a mirror in the brooder with the lone injured keet to keep it company.
Have you candled the other eggs to see if there are any live keets or developing keets in them? If there are, you may want to incubate them... or at the very least be ready to snatch any keets away that do hatch as soon as you see them
Good luck, and sorry you have to deal with the carnage.