This post adopts for sake of argument the forming idea that Sayid shooting Ben causes Ben to become the demented fellow we know and love and hate (also adopting the Ben is Evil theory for purposes of discussion). Say that's true. Sayid has reason to hate Ben, and if it was possible to kill Hitler, you would have reason to. However, Ben is just a kid there, and he is, in fact, innocent. The point of the event making him bad is all about that innocence.
What happened happens, so Sayid is right, this guy is going to do bad stuff. Even if you can't kill him, maybe you have reason to make his life more rather than less miserable, for some sort of justice (not saying I think that way myself but throwing it in there).
Is Sayid to blame for causing Ben? Or is Ben still more to blame? Is there no blame in such a situation?
This is actually a valuable question about morality and responsibility, which can be asked in a more classical way in philosophy, which Lost presents us with.
Me... I think determinism destroys all morality, morality requires true intentionality, and not total freedom of will, but some amount of will power, and some amount of results which can go more than one way. But given the wierds sort-of freedom in lost (maybe you do skip death if only for a day)... I'd have to go on a limb and say Ben is more to blame. In the destiny universe, Ben had it in him. Had Desmond not shot him, his attitudes about his father would have been enough, or some other betrayal would have turned him cold... or he was cold from the start anyway (seems to be the case actually). Sayid in this case is wrong first for not understanding how history works. He is wrong also to change history this way, a better way would have been to have sympathy for Ben, take him to the others, and try to be a good influence on him. But Sayid does not know solutions through compassion, he knows solutions through violence. I wish this show would be about characters realizing how to learn things like that, as it was at the start, and still is, really, but they don't focus on it. To finish with this slight tangent, I would love if by season six people are finally solving their mental problems, not becoming perfect, but progressing past their characteristic flaws of impulse, violence, and short sightedness.