In the one scan with Saix talking to Axel at Memory Skyscraper and he mentions something about Castle Obilivion? Well I think Saix found maybe a tape or some sort of evidence that proved Axel killed Zexion and Vexen and maybe Saix threatens Axel to do what he says or else he tells Xemnas or something else. I dunno it just seems like Axel is going to be in a lot of trouble for killing off two of his elders.