I wouldn't say it's presumptuous, simply because we can't possibly know how longstanding the effects of that one man's actions may be. We change the world through one another just as much as we do through our direct actions - it's why it's important to never forget those we've harmed or victimized, regardless of how inevitable we find whatever it is we've done to them. Even if we're in a position where we can forget them, they're never going to forget what we've done. It works on the same sort of principle.
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