Woof Howie:
Such questions and reflections belong well in this thread.
I would offer for your consideration that animals tend NOT to do the sort of thing we see here. Several years ago I read "The Manufacture of Evil" by Lionel Tiger and Robin Fox who are a team of evolutionary pyschology/biology professor types. Not their strongest piece of work ("Men in Groups" I thought outstanding) because I thought it would have been better served by being shorter, but the basic premise I thought quite intruguing: that evil results from the stresses of man being placed in an environment other than the one in which he was evolved.
Konrad Lorenz explored similar notions (deeper IMHO, but then he was deeper than just about anyone) of the Darwinian implications of when the darwinian selection process no longer involves interspecies criteria (e.g. hunting, getting hunted) or environmental criteria (out running the forest fire, surviving droughts) but instead the only selection processes are intra-species (man vs. man) , , , but I digress.
Anyway, seen through the particular filter of Fox and Tiger, it is no surprise that this incident would occur in what Lorenz would call "the Anonymous Horde" of a Mexico City subway.