The people who committed genocide (or simply murder without cause) are clearly to blame for those they killed, directly and in many cases, indirectly.
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.
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.