That Was Then, This Is Now movie has a happier ending than the book. Mark doesn’t hate Bryon because Bryon didn’t put him in jail for selling drugs, Bryon kicked him out of the house and Mark sped away and was caught by the police. When Bryon went to see him, Mark told him to lighten up and that things would be okay verses the book saying something along the lines of “With that look in his eyes, I swear he [Mark] could have killed me.”
The movie ending was alright, but it just didn’t fit the bill of the story of two teenage boys growing apart. Instead of the two being torn apart by emotions and circumstances like in the book, there was a rift that was sorta patched up at the end. I mean, I get that they might have wanted a happy-ish ending of “family/friends always find their way back” but that’s not always the case and the book showcases that perfectly.
(but hey, at least with that ending we could imagine Mark never ends up the way he does in Tex)