Why does pixiy killed Sibuxiang anyway? Is it because of his brother? Since he said only he left, maybe he thinks his brother died and that is Sibuxiang's fault.
So something happened to Bixie that made him kill Sibuxiang, and to save Bixie Tianlu decided to hide him in his stomach like when they were small. Thus taking the punishment and also throwing them off by saying he's the last one.