Well, I lay out all the evidence in that post, so y’know, just re-read it and see.
But since you want to know why I’m so sure, let me repeat the most damning, undeniable evidence:
Big Walder, the little one, fox-faced and skinny as a stick. His chest and arms and cloak were spattered with blood.
The body in Ser Hosteen’s arms sparkled in the torchlight, armored in pink frost. The cold outside had frozen his blood.
“My brother Merrett’s son.” Hosteen Frey lowered the body to the floor before the dais. “Butchered like a hog and shoved beneath a snowbank. A boy.”
“Where was the body found?”
“Under that ruined keep, my lord,” replied Big Walder. “The one with the old gargoyles.” The boy’s gloves were caked with his cousin’s blood.
—A Dance With Dragons, Theon I
Big Walder claims to have found his cousin’s body. His cousin’s frozen body. With frozen blood, that looks like pink frost. And yet Big Walder is covered with spattered blood — that is, he was sprayed with blood — and his gloves are caked in blood. How is that possible if he only found the body, dead and frozen already?
Answer: it isn’t.