I would not have a problem owning such a hypothetical sword. I already own swords that, based on their age, could have been used to kill but lack the hypothetical bodycount markings to document that history. I also own WWII German and Japanese firearms that were brought back by American GIs that were very likely used to kill Americans. I respect that this would really bother some people, and they would not want to own or even be around such items. I differentiate between the tool and the person using the tool, so I do not view these items as inherently bad or evil. And my interest in them is based on history, artistry, and technology instead of any macabre interest.
That said, I would be less inclined to own a sword (or other weapon) that derives its primary value or interest from the macabre. For example, I would not want to own a test cut sword solely because it is a test cut sword. It would first have to be a sword I would appreciate if it did not have saidan mei. The saidan mei is still valuable to me--albeit secondary value--because it provides additional historical interest. The same goes for kirikomi or Hokke's hypothetical bodycount markings--they provide value to me that is secondary to the craftsmanship of the sword.