A scroll through Thompson's LinkedIn history reveals that many were angry about denied claims.
One woman responded to a post the executive had made boasting of his firm's work on making drugs more affordable.
"I have stage 4 metastatic lung cancer," she wrote. "We've just left [UnitedHealthcare] because of all the denials for my meds. Every month there is a different reason for the denial."
Thompson's wife told US broadcaster NBC that he had received threatening messages before.
"There had been some threats," Paulette Thompson said. "Basically, I don't know, a lack of [medical] coverage? I don't know details."
"I just know that he said there were some people that had been threatening him."
A security expert says that frustration at high costs across a range of industries inevitably results in threats against corporate leaders.
Philip Klein, who runs the Texas-based Klein Investigations, which protected Thompson when he gave a speech in the early 2000s, says that he's astonished the executive didn't have security for his trip to New York City.
"There's lot of anger in the United States of America right now," Mr Klein said.
"Companies need to wake up and realise that their executives could be hunted down anywhere."
Mr Klein says he's been inundated with calls since Thompson was killed. Top US firms typically spend millions of dollars on personal security for high-level executives.