After being violent towards Laurie, what did David realize?

David realized that there are repurcussions for his actions... that his newfound violence is a direct result of the Wave. He comes to understand that his actions are not charateristic of the way he regularly behaves.... he feels guilty for hurting Laurie, who he still loves.

David’s violence toward Laurie further demonstrates The Wave’s destruction of families. It is significant that the only violence that occurs ‘onscreen’ is between a boyfriend and girlfriend (who have considered getting married), and not between friends or acquaintances. This demonstrates the fact that loyalty to The Wave often comes at the cost of loyalty to a person’s previously existing family relationships.