Everything but the microwave was self-defense
Everybody refers to this as a revenge movie. The teaser text on this page says the parents "devise an increasingly gruesome series of revenge tactics". Tactics? They are in immediate danger all the time and commit acts of violence only when they have absolutely no choice. They only really get the upper hand when the movie is almost over. This felt more like a struggle for survival to me than like revenge, which is weird because the rape, happening so early in the movie, is obviously a set-up for the events to come, but it didn't really factor into what happened then at the house. What would the parents have done differently hadn't Mari been raped? Kill Aaron Paul more humanely?
And as the only bit of real revenge, the microwave seemed somewhat random and underwhelming to me.