Except it was not shared, it was almost a genocide.
After smallpox when the population of the Americas had been reduced by something like 90% they most certainly didn't need all the land.
Are you really unaware that the colonials intentionally spread smallpox to the natives? This is not some obscure detail - it's in approximately all of the history textbooks in a fair bit of detail.
If someone acted with clear intent to commit genocide, but the mass deaths would have happened anyway, does that clear them of the charge?
Put another way, if I stab someone, the knife goes in and all, but as I'm doing it a car also runs over him, am I no longer guilty of murder? Seems pretty questionable to me.
As for your example, that's a bit convoluted. Perhaps clearer would be if you intend to genocide say, a town and meteor hits killing everyone before you get there as well as the neighboring town, it would be difficult to argue you're to blame for their deaths. That doesn't mean you're a nice person and we'd generally still lock you up for attempted genocide of the town you attempted to murder, but not of the town you didn't.
The meteor in real life was disease. By some estimates, 90% of the population in the Americas died from diseases the Spanish accidentally introduced by 1600, most the Spanish did not know existed.
Oops!
The east expansion took a lot of time, involved quite a few massacres and invonluntary relocations.