There is no evidence that she ever truly looked inside herself or examined things objectively. Not once. There's nothing as drastic as murdering your lord and falling from grace with your gods. If that doesn't get you to think, nothing will. She was incapable of guilt, either by nature or by choice. Could she have chosen to feel guilt and change? Probably, sure. But Belkar could choose to stop murdering people if he wanted to. Doesn't mean Miko wasn't the most unreasonable person ever.
Change is not such a easy process. Miko barely had any time to reflect on her actions before she died. You compare her to Belkar, but I would rather compare her to Vaarsuvius. Vaarsuvius is also stubborn in the pursuit of her goals, the result of which is that it is hard to make her see things differently. But when extreme circumstances and time do make her see things differently the effect goes a lot deeper.