History is not my avocation, but I think
this is not just a recent thing. It may be
that, only recently, does modern society
expect that mass killings will "never again" happen.

There are common threads.
Excuses: greed, envy, rightousness etc.
Opportunity: weak adversary, unaware adversary,
guilt-ridden adversary.
Other non-violent outlets for solutions appear to be blocked.
Coalescing of ethnic or political groups; and greivances,
scirmishes between groups, that enhance group identity
and hatred of the other group(s).

But, not to worry, there are no examples of this going on.
It can't happen here.