In the Purge, there is an awkward scene where Charlie lets the man from the street to enter their house and at the same time Zoey's boyfriend Henry heads over to speak to James Sanding. We then see Henry pulling out a hidden gun and attempting to shoot James. Who really was Henry and why did he try to kill his girlfriend's father?
Her dad didn't approve of their relationship and would have tried to stop them from being together.
In the world of the movie, people are naturally evil and violent, to the point that the only way to prevent crime is to allow them to get their aggression out once a year. So in that context, murder (on the night of the purge) isn't nearly as big of a deal as it is to us. It's simply an acceptable way of dealing with problems.