It's an explanation, but slightly flawed.
Kenji himself fought in the end, or at least, didn't run. It was sorta impossible to just live, friend going to commit eventual genocide and all. It was because of Kanna's network that as many lives were saved, and Kenji had enough of a distraction to actually achieve his goals. I don't remember anywhere else in the manga him being so harsh or judgemental, period. Hell as a kid he wasn't. He also chastised only her, not any of the group or anyone else.
Also, it wasn't so much of the reproach as the abandonment. "Fuck you, later" was never, ever, ever in his character. He was negligent, careless, and selfish, but he never did anything like that consciously to be a dick. The end implies that was the last time they saw each other.
tl;dr Urusawa got muddled halfway in his story, clusterfuck from then on with extraneous characters thrown in like we shoulda always known them and a retarded ending "twist", that was unrewarding when all the pieces came together.