He behaved like the adolescent he probably was when he had that accident. Imagine trying to recover from a brain injury with that chatterbox of an older sister hovering over you?
Perhaps I am being unkind about that older sister. Sister probably had an ok childhood but used her gift of chatter to shield her from the travails of her current married life of "slaving like a dog" with a banker husband and 3 kids.

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He also met his next potential victim therethe managers wifes friendwho was windowedhe later sees her at the speech nightand refers to her when he is leaving town.
I'm a Lieutenant Detective in, uh, Homicide. That's a fancy name for murder.

He was so rude inside the bank. For someone who wanted to stay with that family, that wasn't particularly smart. I agree however that it was the way his depraved mind worked. He didn't have a filter.

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I don't think reporting was the same then as now. No one would have known outside the bank that money was there. The insulting comments I think emphasized his attitudes about that part of the world. Just like his speech at the dinner table about women with inherited money. Showing how his brain works. As to why he wenthe was still ingratiating himself to the family. Could that be it?