Did Henry And Wakefield kill all the people on the island?

Well, when the FBI or police were talking to shea and madsion they said "You are the only survivors of harpers island, OTHER THAN A HANDFUL OF LOCALS" so I guess they didn't kill everybody

A certain amount of locals must have been killed for them to have been able to do what they did and not get caught. When the whole thing went to hell I’m sure that some hid but Wakefield was out there on his own a lot while Henry was with the group so he very well may have in fact killed many of the locals. Some may have booked it temporarily to the mainland when the wedding guests left since Mr. Wellington dying was vaguely reminiscent of how Wakefield had killed people seven years prior.

As for the ones that were in fact there the people being killed AND the marina exploding was definitely like what had happened seven years prior. It was a close-knit community so it was definitely possible that they all had experienced a great deal of pain when Wakefield had been there seven years prior. As such the reminder would surely cause a great deal of fear and pain. Most people turn around and become very self-serving in situations like that simply because survival mode dictates to take care of themselves and their loved ones and not others. Odds are any of the locals who WERE there and did in fact live hid throughout the entire thing just hoping that they could ride it all out.