Minor example. In the end of the film after The Purge has finished, Mary asked the homeless man if he will be okay. The man nods, Mary thanks him, and the man leaves the Sandin house.
Considering the man has saved Mary and her children's lives in the nick of time, it is understandable. Also, the man more or less considers the family as his savior, for not only allowing him to hide in the house (well, at least Charlie shows him his secret hideout at one point) but also Zoey killing the gang's leader who tried to kill the homeless man.
The Matron Saints. A group who use the Purge not to spread suffering and death, but to protect women and punish those who would hurt them. Even among all the depravity of Purge Night, some people choose good.