That's possible, but the Buddha Boss didn't appear to wield his sword with much speed or power at all. It could just be that sharp, but it didn't appear to dull at all. There are all kinds of possibilities, but we don't know which possibility it is.
As for war crimes, I'm pretty sure the X-gun would fall afoul of the conventions as well. Fortunately for everyone involved, they aren't (or probably aren't, unless Nishi turns out to have been telling part of the truth way back when) official members of an army, and thus cannot be subject to the conventions.
Failure: when your best just isn't good enough.