In the book, he learned that his brothers had been killed (supposedly), and in a moment of despair he slept with a girl and took her virginity. Having already done that, as he saw it, he could either:
1) Leave her a "ruined" woman.
2) Marry her and risk the breakup of his alliance.
In keeping with his father's behaviour, he chose the latter. It was a difficult choice in the aftermath of an understandable mistake.
In the show, he just decided that he can brazenly go back on his word to allies who have committed thousands of soldiers to his cause and risked death for him just because he feels like it. It's utterly unsympathetic. Not to mention that he does this after the Lannister-Tyrell alliance and the defeat of Stannis, which at least doubles the enemy forces he's facing.
It also completely removes the subtext that her family (who are sworn to the Lannisters) pushed her to go "comfort" him in order to achieve the marriage so that they could either break up his alliance and thus get brownie points from Tywin or land in a pretty good place when/if the Starks win.
We've covered that disappointment enough at this point though.
More importantly they somewhat rescued the awful design of the White Walkers from the pilot. Still nothing like they were described in the book, but at least they don't look like zombie gorillas anymore.
I really hope I was seeing it wrong at the end there and he wasn't just letting Sam live for being so pathetic.