Ben said "not too long ago, I made a decision that took the lives of over 40 people. I think the decision he was referring to was to let the others into the Tempest to use the gas.
When Locke brought Hurley and Ben to the Dharma body pit, Ben told Hurley it wasn't his decision, it was their leader's. He must have been talking about Widmore.
When we see Widmore being banished from the island and getting on the sub, the other obviously have control over Dharma's ferry, the barracks etc...This is when Ben becomes the leader. This means it takes place after the purge.