This comment is to answer the question posed by sstocker1 and contains a significant spoiler. I recommend skipping the rest of this comment as well as all of sstocker1's comment if you want to avoid spoilers.
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The man who poses as the son of one of the victims and the brother of one of the cult members is the son of the cult leader. I presume his motivation in presenting himself in those false identities was to better understand his father's actions by being able to get closer to the people who were most impacted by his father -- namely the poisoning victims and cult members -- than he otherwise would have been able to do. I interpreted the burning the pictures of his family and the dock at the end as being indications that he was ready to move on (burning the photos) and that he won't be returning to the lake on subsequent anniversaries (burning the dock).
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The man who poses as the son of one of the victims and the brother of one of the cult members is the son of the cult leader. I presume his motivation in presenting himself in those false identities was to better understand his father's actions by being able to get closer to the people who were most impacted by his father -- namely the poisoning victims and cult members -- than he otherwise would have been able to do. I interpreted the burning the pictures of his family and the dock at the end as being indications that he was ready to move on (burning the photos) and that he won't be returning to the lake on subsequent anniversaries (burning the dock).