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Now, before you flag this question as blatantly off-topic, I claim to have a scientific basis for this(!): not some drug or remote control, but parasitic infection.
While reading about behavior altering parasites and parasitoids, I found this:
Parasites that induce behavioral changes in their hosts often exploit the regulation of social behavior in the brain...For example, Toxoplasma gondii attaches to the hypothalamus rather than target a specific cellular pathway; this broad targeting leads to a widespread increase in host dopamine levels, which may in turn account for the loss of aversion to cat odor...This rise in dopamine levels induces a loss of aversion to cat odor in the rats, increasing the risk of predation by cats, T. gondii’s definitive host.
Now, if a parasite can prevent mice from being afraid of cats, how hard would it be for another parasite to prevent humans from caring about each other's life(!)? Can a parasite develop some method by which it can alter behavior of humans in such a way that they just start killing each other for no reason? How easy/hard would it be for a parasite to do so?
Note: although it is just a curiosity question about whether it is possible or not, giving some description of neural pathways which stimulate social behavior in humans and how they are regulated would be much more appreciated.