I guess it's not really such a mystery why someone would feel driven to kill him. No Big Pharma plots necessary.
Good question about why he sold the company. It sounds from this article
How Barry Sherman built his multibillion-dollar fortune
that Sherman (using a loan from his mother), along with his friend, and his friend's father, bought Empire for $350,000. A few years later they were offered $2 million for it. They each made a whack of profit and dissolved their partnership. Sherman later started his new business without any partners.
I don't know whether he thought about his cousins at all. I think it likely he was solely focussed on making his first million and other people never crossed his mind. He was definitely not the kind of person I'd trust to take care of my interests.