what is ironic about David's violence toward Laurie?

David is actually Laurie's boyfriend. They seem to be the perfect popular High School couple until Davis gets swept away in the Wave. He is very kind to Laurie and initially even defends her when she resists the Wave. After Robert threatens her David says,

I don't like Robert's attitude," David hissed back. "It's like we must wipe out anyone who resists us. That's the exact opposite of how we should approach this."

Still David does get so caught up in the Wave, he finds himself eventually doing the opposite, he grabs her wrist and throws her to the ground.