He only changed because of the magical powers Bioware protagonists have on other characters. He showed no intention of changing when we met him, being more than content to live the lie he was living for the best of his life, regardless of the consequences that could cause. If he wanted to atone and be a better man, he would have gone to Orlais or the Wardens regardless of the fate that awaited him.
I'm not sure "content" is the right word. "Resigned" might be it. He didn't assume Blackwall's identity just because it was convenient. He did so because Blackwall was worthy of life and he was not.
Though yes, fear of death did play a role. He should have gone on to the Wardens. But then, he had no way of knowing they would have accepted his story. But in the end he overcomes even that and publicly confesses his crimes.
He has yet to redeem himself in my eyes. He has yet to make or even attempt to make things right with everyone he's wronged.
Have you seen his non-Warden epilogue in Trespasser?
Thom Rainier was shown mercy when none was deserved, and set on a path of redemption. This gift, so compassionately given, needed to be shared. Freed from his obligations to the Inquisition, Rainier traveled Thedas, giving hope to the condemned and the forgotten. In the deepest prisons and pits of Thedas, he found, if not goodness itself, its potential. By showing faith in those who had none, Rainier lifted them up and made them into something better than they were.