Hallo Johnny,
In treatings on kamma (buddhist kamma) the existence of such a thing as ‘collective kamma’ i have seen being denied, for example, by Dagpo Rinpoche in his book named Karma.
I do not know what is the theravada doctrine. Is that massacre been explained as collective kamma? of the Sakyans?
That individual choices or intentional actions in body, speech and mind, in some way or the other, can become collective, and spread, as it were, over other people too, that does not look as a buddhist doctrine, but maybe i am wrong.
Siebe