How is the Dr. killed? Why does Muff Potter think he did it?

Injun Joe kills Doc Robinson with Muff Potter's knife. Muff was drunk and doesn't remember the fight.... thus, Injun Joe tells him that he (Muff) killed the doctor.

"I thought I'd got sober. I'd no business to drink to-night. But it's in my head yet -- worse'n when we started here. I'm all in a muddle; can't recollect any- thing of it, hardly. Tell me, Joe -- HONEST, now, old feller -- did I do it? Joe, I never meant to -- 'pon my soul and honor, I never meant to, Joe. Tell me how it was, Joe. Oh, it's awful -- and him so young and promising."

"Why, you two was scuffling, and he fetched you one with the headboard and you fell flat; and then up you come, all reeling and staggering like, and snatched the knife and jammed it into him, just as he fetched you another awful clip -- and here you've laid, as dead as a wedge til now."