>>214407626
They explain it just fine.
>Melissandre shows him visions in the flames
>births Stannis' cum demon to kill Renly
>3 leeches with Robert's bastard's blood kills the 3 people he names
By this point he just straight up believes anything she tells him because she says he'll win the Iron Throne and "oh the lord of light said you gotta do this to make it happen" without second thought. He's blinded by ambition and has enough proof in his mind that she actually speaks for a god so when this woman (and therefore god himself) says he has to sacrifice his daughter or he'll lose, he obeys. Plus they didn't have enough food to get back to Castle Black and were losing men to the cold, or they were just deserting his army so I imagine his thinking was that they were all going to die anyway if he didn't do what he did.