I don't think there's any original sin in FF7. Unlike in a lot of other games, where there is one bad guy who is definitely behind all of the other bad things, FF7 has quite a few baddies who work semi-separately. Hojo is certainly behind the problems that relate to some of the characters (Cloud, Sephiroth, Aeris and Red XIII), but Shinra in general is still pretty evil. Assuming that they would still have taken over the whole world without Hojo's SOLDIER research (I think they would have; it would just have taken longer), they'd still be draining the planet of Mako energy, making them the biggest baddies in the wider scheme of things, and they still would have been responsible for all of the bad things that lead to the creation of AVALANCHE (unless I'm missing something that is revealed in Before Crisis). The bad things that happened to Barret, for example, don't have any special connection to Hojo.
There's also Jenova and Sephiroth. Jenova has a lot of "Original Sin" qualities (moar like "Original
Crisis" amirite?
); she did kill all of the ancients, after all. Jenova had the biggest impact of any baddie on the world of FF7. Hojo used her, but one might say that she used him as well. Sephiroth is, of course, thanks to Hojo, but also thanks to Jenova, and he has enough freedom (from Hojo, but not necessarily from Jenova), for one to say that Hojo is not really the core of the Sephiroth problem. After all, Sephiroth's actions weren't controlled by Hojo, and he did the opposite of what he was intended to do. If Hojo had said "
just as planned" when Sephiroth went crazy, we might be able to say that he was the core of the problems in FF7. But he didn't. So there.