"One-shot" is manipulative terminology because of all the conditions that are ignored, mainly that it was a surprise attack, and Zoro was still in a non-serious mood from his previous encounter with the wacky guy (forgot his name) who distracted him in that moment, and Zoro only turned completely serious after that moment, when he instead one-shotted and completely overpowered a non-distracted Killer.
Afterwards, Zoro seemed to have lost some of his fighting spirit, because he had ended the fight, which probably contributed to his passing out, the same as it does to Luffy. (Edit: It seems the hunger is an important canon point.)
Oda has however has no obligation to shit on characters, although people always expect him to shit on someone for someone other's benefit (just a bonehheaded way of interpreting stories), and Killer being someone of a similar rank as him in someone else's crew is still displayed as strong, and when he damages someone, it can have serious consequences. His name is also "Killer", so I suppose he uses nasty techniques which either cause immediate killing (which somehow is not a convention in Oda's world, even with swords) or bleeding. So I suppose the effect had something to do with bleeding. (Edit: or more likely hunger.)
I don't think he uses poison, partly because I think Queen already uses poison in this arc instead. And he seems a brutal, "edgy" guy normally (even if his experiences made him "sad"), so I bet he is all about slicing and stabbing opponents. And I don't think poison hypes Killer more, or that there is any issue with his strength level, which I would put easily in the Tobi Roppo range, for example.
(Edit: I guess I might be wrong about Queen using poison, I looked up the chapter 970, Kaido vs. Oden again, to check on whether he had a mechanical arm. One can't see that arm, however there is an aura on his weapon which looks like poison, and I wasn't sure about what he had used for torture anymore.)
The scene is supposed to highlight a "new" character or one which we didn't really know before, so Oda won't discard him by some lame feats, or sh-- on him, and he won't just "wank" a main character either (he actually does that rarely, only when there are no stakes, or he has gone so far to fake-kill them already).
If in doubt, I just go by what happened, and don't bother drawing hard lines or overemphasizing someone's superiority. What one has to keep in mind here is Zoro's high potential in defeating enemies, it doesn't mean he can't possibly be harmed by someone else who is clearly "strong".
Anyway, this thread was supposed to be about the use of poison, so I don't understand the "shitting on" characters introduction.
