KIller would have been food poisoned as well. He was an outlaw long before Zoro became an outlaw. People think Killer ate at fancy diners?
Killer was out of his mind, and his fighting style revolves around agility and momentum with the rotating sickles, generating torque with his body to land power moves, not as a QCQ with a scythe. He had to learn to fly with scythes on the fly and still kept up with Zoro.
Like people dont realise how hard it is to fight with a scythe against a sword user if you're not an experienced user. A scythe is a much less sturdy weapon than a GRADED sword, plus the scythe is so much longer, he has to apply more force to counter Zoro's attacks (ie the mechanical concept of a lever arm) so Zoro will always have the power advantage. Killer has to be much stronger physically than Zoro at because of that and because the scythe is not as sturdy as the sword
The ONLY benefit the scythe has is the long range, but to leverage that he has to have better skills than swordgeek Zoro, and he has to learn to wield the scythe on the fly
Killer is just simply better than Zoro. This chapter confirms