Presumably, at this point, there's a 99% chance this "karmic demon" will be related to Maria (and my mind goes in all sorts of directions when thinking of the possible reasons as to why rat-boy would have her child; none of them particularly pleasant).
Since we know Saki, at least, somehow survives this ordeal since she's narrating the whole thing, it's good to know she'll have time to self-reflect on how she inadvertently caused so much death.
Then again, Saki did insinuate it was Maria's fault way back in the beginning of this series, which doesn't sound like the statements of someone who's done much self-reflecting, so perhaps narrator Saki-much like she and Satoru were in this episode-is blind to what seems rather obvious.
As for killing a human with your Cantus, I would assume it's still possible, as long as you're willing to give up your life in return, a realization I'm sure will occur far later than it should. In keeping with the pretty prose of the story so far, I wouldn't be surprised if Satoru eventually makes this sacrifice, and only then does Saki finally realize she's just lost "the one".
