There was a point after Half-Blood Prince where he could choose to change. There was a point where he had to see error of his ways between that and the Battle of Hogwards. It seems he took that step for a while, there was redemption arc brewing - but never happened.
He was indocrinated, yes. But he saw how terrible their side is and still chose to stick with it when he had a choice. He was 17 and adult in Wizarding World. Old enough to know right from wrong.
It is likely that some time after, he regretted it. Otherwise I don’t see Harry and him nodding at one anoyher in the epilogue. But at the time of the books, he was not a good guy at all.