Could we all become murderers and rapists...

Not like what you see in this movie. Not even close. Undoubtedly there are some people who would, but no society could function if everyone were like what you see in this movie. If everyone were willing to be this murderous and cutthroat, society couldn't function the other 364 days of the year. In this movie, you see the main characters -- nice, upper-middle class people -- shocked when their neighbors turn on them in the purge and try to kill them. I assure you, all your neighbors, whose backyard barbecues you attend, whom you go to ball games with, whose kids go to the same schools as yours, and who go the same civic league and PTA meetings you do, don't all secretly want to kill you, and smile malevolently as they watch the light go out of your eyes. That's true sociopath level coldness, and most people aren't like that.

In the real world, violent sociopaths and psychopaths have lengthy histories that point to what they are. The smartest ones can do a good job keeping those histories hidden, but normal, honest, productive, taxpaying, law-abiding people aren't all secretly cold-blooded thrill killers under the surface. If they were, they wouldn't be normal, honest, productive, taxpaying, law-abiding people.