He did more than walk away from the old Jedi Order. He walked away completely from everything and everyone.
Because he is a flawed character who thought he was too dangerous to do any good. It’s kinda the necessary foundation for his character arc.
I’m almost willing to forgive the Luke tempted to kill Kylo flashback stuff, but what will still remain a head-scratcher to me is buying the idea that post Jedi Academy-Luke doing absolutely nothing, NOTHING, to use his time on the island to figure out a way to redeem Kylo or help in any capacity, especially when he hides little clues like a piece of the map of his location in R2D2. I feel like an ashamed Luke will guilt him into action; not “leave me to die” Luke.