How could it be? Well, I think it is quite simple. Consider this recent text from climatologist James Hansen:
"Extreme climate anomalies such as the Texas-Oklahoma heat wave of 2011 and the Moscow heat wave of 2010 have increased in frequency to such a degree that they can be ascribed to global warming with a
high degree of confidence."
And according to some estimates more than 15 000 people died in Russia only as a result of this heat wave.
And we of course know that anthropogenic emission of greenhouse gases (GHGs) is the dominant (anywhere between 100 to +150 %) cause of global warming over the last half-century.
And since practically everyone in the (developed) world emits some amount of GHGs, then everyone in the (developed) world is to some extent to a mass killer.
Oh, but wait. Also all those people, who prematurely died, emitted some amount of GHGs. We we are not only (mass) killers, we are also suicides.