Once Arthur's burial and funeral had concluded, Alaric had gone into a withdrawal. He returned to the academy a day after everything, and even though his mother begged him to stay, he simply ignored her words and left. He had gone through the passing of his first friend by wanting to be surrounded by people he trusts, as he knew what might happen had he told them the truth.
Yeah, at first he figured that people would try and tell him that it wasn't his fault, especially his parents, because of course they would right? If your 19 year old son blamed himself for the death of his best friend, a good parent would probably tell them the opposite, that it wasn't their fault.
