I would agree if the "disease" in question started with the introduction of market style capitalism, but that is simply not the case. People kill for greedy reasons in traditional, command, market, and mixed economies all the time. It is nothing new.
It can and has happened in capitalist societies but it is not exclusive to capitalism.
Stating that Murder due to greed is a disease of capitalism gives the notion that capitalism gave birth to murder and greed. Murder for greed is as old as humanity, think about all the wars fought over the span of human existence for money, power, land, women, slaves, and even just because of personal dislike. Did Ghangis Khan invade all that land and murder all those people because of capitalism? How would that be possible thousands of years before capitalism was even invented?