Well, you see, a real sword couldn't really kill multiple people at once, or rather, with a single swing. See, when the blade would hit the first target, it would burrow itself inside the enemy, likely not cleaving through completely. This would be even more likely, if it was in the gut area, or anywhere near the chest, as the blade would have to pass through more, er, "mass". However, lets say you cut through a thinner part of the body, say the neck. And you put enough force behind that swing so that it can completely cut through the victim. The blade would continue to go on, but the amount of force that was exerted onto the first subject would severely decrease the speed as to which the sword would be traveling, and thus would drastically decrease the chances of the sword being able to deliver a killing blow on the second target. And this is not factoring in the likelihood of two people being next to one another for such a swing to be feasible. It's still possible, but not as likely as you would think.
This is a repost. The site glitched and I have to repost this. :(
In pretty much every other game, this is also broken. I would just say that William has superhuman strength and is able to slice through many different people(even if he doesn't look super bulky, like Rotondo) in a single swing. We could also say that the sword is magic, and considering it makes the bodies all light blue and unrecognizable, it probably is, but anyways, and is able to slice through many people as if a ghost might pass through objects, and can damage/kill the victim(s). In other games as well, you sword may pass completely through the enemy's body, with the blade sticking out the other end, and the enemy only takes damage, and isn't cut in half. Also take into consideration that when two basic planes crash into each other, they spas out and do all sorts of flips and turns in this game.
To put this in a sentence, this game's logic and physics is already completely broken and pretty much has always been, as far as I know and have experienced.