Marianne thinks cruelty does not only hurt the victim, but the perpetrator also, and maybe more deeply and more permanently. You learn nothing very profound about yourself simply by being bullied; but by bullying someone else you can never forget.
After the funeral she spent evenings scrolling through Rob's Facebook page. Lots of people from school had left comments on his wall, saying they missed him. What were these people doing, Marianne thought, writing on the Facebook wall of a dead person? What did these messages, these advertisements of loss, actually mean to anyone? What was the appropriate etiquette when they appeared on the timeline: to 'like' them supportively? To scroll past in search of something better?