Definititon of Purge:
In history and political science, to purge is to remove people considered by the group in power to be "undesirable" from a government, political party, a profession, or from community or society as a whole, often by violent means

Mikhails telling of the event on the island referred to as the Purge is a very interesting development. Of course, he was being misleading in portraying himself as a Dharma participant, when in reality he was most likely one of the "hostiles". But it seems as though such an event did occur...although who initiated it? Did the others/hostiles see the DHARMA'S as a threat to thier community and resolve to end it? It would appear that the DHARMA'S were wiped out. Have we seen any actual DHARMA participants other than Roger? Ben has referred to his group as the good guys, he has been on the island his entire life. They have taken over some of the hatches but really do not seem to
have a connection to them. They do not even like being there, and do not consider it home. So, it is logical to assume that Juliet was not recruited by DHARMA, but by the others/hostiles. There has been a war being fought on this island. Many of the participants have been lead to believe that the outcome will impact the entire future of the world for good or bad. Most interesting was Mikhails statement that the hostiles had already been on the island for "a very long time".
Were the developments that would be made by DHARMA, going to eventually lead to tragedy and they needed to be stopped in order to save the future world? I know the whole time travel thing is supposed to be out....but it sure seems as though the "hostiles" may have been put in place to stop the DHARMA'S long before they got there.