Spectrum,
the first obvious assumption that came to him was they got what they deserved.
If his assumption was fairly represented in your first post and was based the fact that they sold off and got to preaching, then he assumed a motive not stated in the premise, as well. In other words, the first obvious assumption was very likely flawed because it was an assumption that included a detail not included in the facts presented—motive.
Unless you misrepresented his statement in your initial post. Or unless you are misrepresenting the facts you related that served as basis for his assumption.
He said "They deserve it. They thought that they were going to get extra brownie points from God for giving up their comforts for a short period. It should have been in their hearts from the beginning to save people and not try to make a big show of it to God right at then end."
Let's take this in parts:
(1) "They deserve it. They thought [fill in the blank with whatever motive you like]."
Please explain how your nephew knows what they thought. If your 17-year-old nephew doesn't know what they thought, then he assumed what they thought, which he presents as motive. The motive he assumes was present is an ulterior and a selfish one.
(2) "It should have been in their hearts from the beginning to save people and not try to make a big show of it to God right at then end."
What would you do if you genuinely believed the world would end next year? Are you actually going to try and convince me your life would not change at all? Your priorities would not change at all? What about your nephew?
Was it not in their hearts "from the beginning to save people?" How did your nephew come by that knowledge? His remark was incredibly judgmental and terribly misinformed, in my opinion. He assumes they were trying to make a big show of saving people right at the end, when they might have simply felt freed up to do so more fully since this old system is ending next year anyway.
My only challenge to what your nephew said is it evidences narrowmindedness in his application of reasoning skills, a penchant for excusing his precious organization for any actual HARM they cause, and a leap to a prejudiced conclusion. There is a highly plausible alternate viewpoint, but he chose the one that criticizes the lowly adherents rather than the errant prognosticators (false prophets) who held themselves above others.
Respectfully,
AuldSoul