When Ben runs into Richard in the jungle in TMBTC, he asks Richard if he is a Hostile. Richard chuckles and asks Ben if he even knows what that means...

I think Widmore's group were the hostiles.
The war between Dharma and the Hostiles is still happening.
The Purge isn't what we think... perhaps it was a sacrifice the island demanded or another DI experiment.

another DI experiment.
Thats a pretty bad experiment, it killed them all, this is interesting, I don't think it's correct, but still very interesting, on second thought maybe it is correct, although Goodspeed calls the Hostile's Natives so maybe there are 2 groups of people on the Island, or I guess that would make it 3.

I had the Hostiles down as anyone who was there before DHARMA arrived, "the Island's hostile indigenous population" as Chang put it.

How would Widmore's group being the Hostiles reconcile with the fact that Widmore financed The DHARMA Initiative?

And by Widmore's group do you mean...

...the "our Island" army guys? There's a spoiler that we'll get to see a young Charles Widmore, under the command of Richard Alpert?

Yes, I mean the army guys are Widmore's guys. Well, RicahrdStone: your spoiler is a clue then perhaps the Richard Alpert part of my theory is wrong, but it also strengthens the idea that the Hostiles are indeed Widmore linked.

And since Widmore funds the DI, it strengthens the idea that the Hostiles were part of the DI all along playing the role of indigenous people that posed a threat to the Di in some kind of experiment.

Remember TLE in which Mittlewerk wanted to sacrifice 30 percent of the population of whatever islands... in order to end hunger, overpopulation, etc. Way harsher than the Purge.