Eh, that just makes it seem like a lie by omission. I still think that he must be at least somewhat convinced by Melisandre’s prophecy talk, and find it to be a plausible explanation, or else he would shut it down.
Abandoning his principles? Where? He kills his brother, aye, but his brother is a traitor. He “cheats” on his wife, but not out of lust but rather out of necessity. You could probably even argue that Stannis is asexual. And then what happens in the show to his daughter is bullshit and I see no way that book Stannis would do that. Of course, that could still be defended by claiming that the ends justify the means. Essentially, would you rather kill your daughter or lose World War 2? Still, bad writing from the show there.
Stannis is a loyal, dutiful, just man who does not abandon his principles throughout the books, and that is what makes him special. He is bound to lose eventually, partly because when push came to shove he cared more about winning the right way than just winning at any costs, but he will give Westeros a run for its money and show them how terrifying a truly just man can be.