What incidents in That Was Then, This is Now point to the fact that revenge is pointless and wrong?
If I were you one of the incidents I would turn to would be when Bryon is beaten up by the Shepards for what Mark did to Angela when he cut her hair as punishment. This occurs in Chapter Eight, but what is really interesting about it is what Bryon says to Mark at the end of the chapter. He asks to speak to Mark on his own, and when they are together, he says to him that he doesn't want anybody to fight the Shepards in revenge for what they did to him. He goes on to explain his reasoning:
I don't want to keep this up, this getting-even jazz. It's stupid and I'm sick of it and it keeps going in circles. I have had it--so if you're planning any get-even mugging, forget it.
Note the way that Bryon talks about revenge as a process that "keeps going in circles" and as something that is "stupid." Certainly the cycle of violence that the novel presents is something that doesn't help anybody, and only hurts those involved in it again and again. This is a truth that Bryon, through his experience and his relationship with Cathy, is coming to realise, though Mark, of course, has not discovered this truth for himself as yet.