It was very out of character, even for the show. Show stannis still took sacrificing people as a distasteful thing that may be necessary.

Shireen being burned in the book literally can't happen how it did in the show. Shireen, Selyse, and Melisandre are all at Castle Black while stannis has been off getting more men and heading to Winterfell. Stannis is actually on the up and up, will likely beat the Bolton's with the hinted at trap plan he's got. Things are so fucking different.

And yes, of course you can say them drastically changing him more than other for season 5 with a plot line that was panned by reviews for being an unrealistically depressing attempt to throw increasingly bad things at stannis, bordering on comedically so, is character assassination.

The whole thing was just absurd.

Then again, listening to D&D's commentary on the season 5 finale actually contributes to the pretty solid idea that they change things specifically to satiate a fetish for no-dialogue scenes of people emoting. They just wanted to film the actors grim face... Most of D&D's detractors wrongly credit to malice what can better be explained by stupidity.