Nothing wrong with a good redemption arc. I imagine being able to change one's destiny being such a common refrain in fantasy and adventure, also plays well into stories about changing your alignment.
We're also probably engineered to consider stories with a good ending, more satisfying.
It would be interesting to have a companion start good and turn bad at the same time another companion is starting bad and turning good.
This is how Woody Allen movies work, and it's pretty fun to watch.
