Scipio I think that is an excellent post and I see it exactly the same as you. Had the Bambers been poor they would all have lived.
If they had been poor and thus he had to work elsewhere he would only have seen them infrequently. If they hounded the crap out of him when he visited- challenging him to make something more of his life then he simply would have be able to visit less.
He had no drive to go do anything else and thus agreed to work in the family business with the understanding he would take it over when they die. He didn't like the business though and didn't like working period. He could have left but if he left he would have to find a job elsewhere and share the inheritance with Sheila after the last one died.
The easiest way he could get instant money and be able to stop working was by killing them. He would have to pick some grand robbery scheme (like a bank robbery) if they were poor and he was still hell bent on instantly becoming rich to quit working. Killing them was a grand crime realistically attainable as opposed to pie in the sky crimes like drilling into a bank vault.