You could do more with the opening line to draw people in. This might also read a little easier if you started new paragraphs when people talk. Obviously, it's a stylistic choice you made, but that's what I would have done if I wrote this, although it might make it look to chopped up.
The otaku was shocked and frightened whispering 'How? What happened?'.
I'm not too sure about this sentence. What I think you mean is while the devil is questioning the killer, the otaku is standing around, kind of dazed. It doesn't make too much sense if this is the otaku's direct response to the killer admitting to what he did. Either way, it could use some improvement.