Some further observations...
Between his flashbacks and his actions since arriving on the island, Sayid seems to be caught in a vicious cycle of torturer and tormented (he tortures that woman in an Iraqi prison, lets her go, flees, gets caught out by her new hubby, tortured himself, is let go in turn, and is on Flight 815 for the express purpose of finding her again). And since arriving on the island he has resumed this cycle of torturer and tortured. Eventually, Sayid is either going to end up dead as a direct result or this or he really will have to swear it off.
Then there be the `incident' (late 1970's), the `purge' (early 1990's) and the `sickness' (mid 1980's).
Lessee...Dharma sets up camp on the island in the early 1970's. At that point in time, they think they have full control over maybe half the island, and a deal of some sort with the pre-existing inhabitants. They build `open' stations on their half of the island (Barracks, Flame, Hydra (even though its offshore) and the docks). But Dharma is very interested in something on the half of the island they don't control, and set up a set of clandestine stations to investigate (Swan, Staff, and Pearl).
For a time, this setup seems to work. Then comes the `incident'. Whatever the `incident' is, it is devastating to Dharma's projects, especially the clandestine ones. It also turns the islands other inhabitants against Dharma, at least somewhat, turning them into the `hostiles'.
At this point, there must have been some sort of `hidden war' or `cold war' between Dharma and the `hostiles'. Then comes the sickness - the plague that killed most of Danielles team (and probably a lot of other people) - a plague that Dharma had the cure for. Conclusion: either Dharma released the plague intentionally or it was something that got away from them. Given that the Dharma project was set up to research the end of the world, and plague is certainly one way that could come about, well...
For the `hostiles', the `sickness' was probably the last straw. A sort of `on again/off again' conflict became something much more serious.
Now...at this point, the bunch we call the `Others' must have entered into the picture. Unlike the `hostiles' they were interested in remaining connected with the outside world, and appear to have major influence/sponsors there. At a guess, those sponsors would have been ...allies turned rivals?...to Hanso himself. At any rate, they became aware that the island existed and that there was some very interesting research that had been conducted there. So...possibly against Hanso's wishes, and maybe without his knowledge, they went to the island and checked it out.
They became aware of the Dharma remnants and their war with the `hostiles' almost immediately. Probably, they didn't get along to well with the Dharma leftovers. My guess is they made an alliance of some sort with the hostiles, and between their tech and the `hostiles' knowledge, were able to wipe out the remaining Dharma presence, and were able to do so in such a way that Dharma mainland HQ concluded it was the `hostiles' acting alone. This was the `purge'.
Then the `Others' moved into the Dharma facilities and took over, concentrating most of their attention on the `Hydra' station.
But...if this is so...what became of the original `hostiles'? Were they all wiped out in the purge? Did we see some of them early on in season 2, when Jin and Eko watched the barefoot band go marching by? Are they the whisperers? Are they still in some sort of alliance with the `Others'?
Something to think about: early on, the `Others', acting on what they knew of the lostaways, dressed pretty rough. Possibly this was because this is how the `real' hostiles dress and they thought it likely the lostaways had run across them a couple of times?
I dunno. Looked at one way it simplifies things, in another way it makes matters more complex.