The Warden and the counselors
It's pretty easy to guess who the bad guys are in this one. When you've got a bunch of twisted grown-ups making kids do hard labor out in the hot sun day after day, all in greedy pursuit of a buried treasure, it's not hard to figure out where your loyalties lie. The Warden may be the ringleader, but the counselors and the whole camp system certainly contribute to the cruel treatment suffered by the boys.
P.S. Don't forget about the curse: do you think that could be an antagonist, too? How?