Is there really anything stopping an evil government from just poisoning the water supply to commit a massacre/genocide/ethnic-clensing?

Random thoughts just popped in my head and I just remembered that all the water in a given city is all centralized. Just add poison and boom, eradicated an entire city who are full of dissidents/opposition as punishment for voting the "wrong" way, or for a protest, or if the population is filled with "filthy undesirable [Insert Racial Group Here]", or something like that.

Kinda unsettling to see that we are all just attached to "the grid" 😖