At first glance I assumed Shizukiri was the killer referred to in the title here--after all, he does kill Mayumi. But then I remembered that one of the reasons Usagi gave a couple of issues ago for abandoning Mayumi was that danger follows him and he didn't want her in harm's way. And he tells her in this issue that he brought her only death. So the killer could be Usagi, in his own guilty view. Or it could be the life he leads.

Whoa. Google tells me it was in sketchbook #4.

Hmmm, tough to say which one I like more; both are pretty strong though I think the original is perhaps more emotionally charged.

I think that if there is one flaw to Usagi it's that occasionally the consequences of the stories don't seem to have lasting effects. It's not a huge glaring fault, but I think it's an unintended consequence of highly episodic storytelling.

The form definitely follows the purpose, which is pretty ingenious, but I still think it lessens the emotional punch a bit because although the losses are powerful when you're reading them that impact fades the further out you move from them. Part of that I think is due a lot to the simple reason that Stan's epic has been going on now for like thirty years now so the size alone is going to numb some of the consequences, but the extremely episodic nature compounds that size issue as rather than say twenty large, consecutive stories over those thirty years you have a hundred stories which while it really immerses you in that wandering feel it does make things feel a little small on occasion when viewed in hindsight.  
As I said, I don't think it's a huge flaw; but rather perhaps a small limitation of an otherwise ingenious form.