Unless this guy has a wife and kids whom he feared the Worm would harm if he failed to accomplish his mission, I do not understand the motivation here, but I certainly hope something will be provided to justify the lamest fight we have witnessed. Though really, it should have been provided in advance so that we could actually enjoy this moment of sacrifice instead of finding it utterly retarded.
What's worse is that I don't even see how he has accomplished his mission in any way. Naidan put Ryuki into a kill or be killed situation, and Ryuki is crying about the whole thing. Ryuki is still clearly conflicted about killing and not happy about this. Unless by some incomprehensible reasoning following this event Ryuki decides that after all he's just a killer so it's best to run away and keep on killing.
It's too bad they don't have Hanafusa around as he would undoubtedly be able to patch up Naidan. Maybe he will show up next chapter and be like, hey, if it's not dead, it's a win for us, right? Then Naidan will open his eyes and be like, hey this is the azure skies but no it's Hanafusa who's like, can I dissect you now?