I think what he meant here was that he believe that his FRIEND wont kill him, so if he is executed it confirms that his friend is irredeemably corrupted, and pretty much not the same person anymore
@SunWukong I think Rahu will join forces with the Sanzhang's crew. If you read Feng Shen Ji the Dark Ones joined up with Ah Gou's group as well. These types of stories are always about the under dog earning trust from new comrades to wage war against the authoritarian rulers. I'm pretty sure Rahu and the Asurans under him will play a huge role in helping Bai Lang's group. On Rahu he probably saw that the current state of the Asura realm is not good and probably notice some conspiracy connected to Heaven and wanted to fix it. But then again..I'm just basing this off of what I read in other manga made by the writer/artist.