In this scene, the leader of the boys, Sebastian, was chasing down Colin for breaking the Sinner's Sorrow. The Sinner's Sorrow was the idea that made Sebastian feel powerful, so he started to chase him down to punish him.
Sebastian caught up to Colin, but Colin locked himself away from the others. Sebastian threatens Colin that he was going to hurt him. but Sebastian could not open the door. He ordered the others to stand guard and also others to go find the keys.
In the final scene, they got the door open to go find Colin. They are trying to find him to punish him. This scene is important to the rest of the story because it shows irony that the kids needed to turn into the adults of the island to keep everything moving. This was a necessary changed that they did not want tot do.