Cassian's line about the terrible things he and the other volunteers had done for the Rebellion had me thinking about Kallus. He certainly did terrible things for the Empire and maybe after he defected he did the same for the rebellion.

So when Cassian asks for volunteers for what's essentially a suicide mission Kallus signs up for it, seeing it as his chance at redemption.

+if he leaves some kind of message or says goodbye to the Ghost crew
+++(hints of Kalluzeb are always appreciated but not a dealbreaker)