Tim Pierotti, who once ran a consumer hedge fund for Martin Shkreli, said he had already lost faith in his boss by the end of 2012.

Then a letter from Mr. Shkreli came to his home, addressed to his wife.

“Your husband has stolen $1.6 million from me,” it read.

“Your pathetic excuse of a husband,” the letter added, “needs to get a real job that does not depend on fraud to succeed.”

“I hope to see you and your four children homeless,” the letter said. “I will do whatever I can to assure this. Your husband’s arrogance is infuriating, and making an enemy out of me is a mistake.”