I agree the argument as to whether ot not Superman (in any incarnation) is capable of killing has been done nearly to death.
But DC Comics (the owners) did tackle it a while back and came to the conclusion that if the provocation was great enough, even Superman would kill. (He killed Zod.) He then would have to face his own demons at his failure to live up to the standards he'd set for himself.
Which, to me, seems to be what this particular story is about - having done the deed, how does he and those around him deal with the demons that have been unleashed.
