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Interesting. I'm definitely on the left rather than on the right, which is consistent with what you say; but I have to admit that I don't see where Lalartu says or implies anything about whether Earth will actually deserve any blame for the colonists' misery. (And, not so consistently with what you say, my own opinion is that if the colonists freely chose to be colonists and the home civilization on Earth didn't do anything terribly awful to them, then if they're miserable they shouldn't blame Earth.)
I'm mystified by some other features of Lalartu's speculation, though. I don't see why we should expect any colony's existence to be miserable, at least once it's past the earliest struggling-to-survive stages that it might well face; I don't see any good reason to expect that the colony -- especially if it's struggling to survive -- would want to nuke Earth; I see still less reason to think they could nuke Earth hard enough to cause anything like extinction.
Lalartu simply says that the colonies will resent Earth, which rests on the unquestioned presupposition that the colonies will live in misery.
I agree that the colonies should not blame Earth for any harm they do to themselves, but, from reading Lalartu's tone, it seems to assume that Earthers can do no wrong.