"Swords and Kunai are lethal" is a non-argument. Those are muscle-powered devices, they scale in power in direct proportion to the physical and metaphysical might of the wielder.

Similarly, the knife-trap is a non-argument. That's exactly the same as saying "nails are lethal because you can put them in a shrapnel bomb!" Well duh they are. That's not representativw of what a normal human can do with such an item.

In all three settings, we have examples of people being threatening with mundane items because of extraordinary skill and superhuman might. In all three settings we also have examples of characters who are utterly immune to same items when wielded by normal people. One Piece actually has most such examples because it has most scenes of mundane people trying to stop the protagonist.