I've thought that the kittens are so fearless that it is
unnerving to adult cats. Large cats sometimes kill
the litter of other males but I'm pretty sure it doesn't
happen with domestic cats. We've had so many cats
that don't get along at first and then end up fiends.

You might try putting them in ajoining cages or plastic
boxes with as much contact as possible without them
being able to actually bother each other. They might
get used to each other. A lot of time they just work
things out in some mysterious way having to do with
pecking order or something like that.

Good luck : -)