Mmm...Remeber in the Half Blood Prince, Dumbledore said why he had trusted Snape....

"He said he was sorry that he had ever done it, and I beleive it was the greatest regret in his life"

Well obliviously it was'nt, mmm. This is tough. Maybe in his trial he did occlumensy and said everything that the trial was wanting him to say. Or maybe he got the panel Imperiused. Anythings possible!