UnitedHealth Group CEO Andrew Witty mourned the killing of executive Brian Thompson and said he understood public frustrations with the "flawed" U.S. health-care system.

"No one would design a system like the one we have. And no one did. It's a patchwork built over decades," Witty said in a New York Times opinion piece on Friday, his first public comments since Thompson, the CEO of UnitedHealth's health insurance unit, was shot dead last week.

The killing has ignited an outpouring of anger from Americans struggling to receive and pay for medical care and has called fresh attention to deepening resentment over health coverage.

Witty said he and his colleagues were "struggling to make sense of this unconscionable act and the vitriol that has been directed at our colleagues who have been barraged by threats."

Americans pay more for health care than residents of any other country, and data shows spending on insurance premiums, out-of-pocket costs, pharmaceuticals and hospital services has all increased over the last five years. It can lead to unexpected costs for care people thought was covered by their health plan.

"Health care is both intensely personal and very complicated, and the reasons behind coverage decisions are not well understood," Witty said.

"We share some of the responsibility for that," said Witty, adding that his company was willing to work with governments, health-care providers and pharmaceutical companies to find ways to "deliver high-quality care and lower costs."

Insurers say they work to negotiate down increased fees from doctors and hospitals, as well as costly prescription drugs and medical devices.

WATCH l Wired reporter David Gilbert unpacks online reaction to shooting suspect:

Why is Luigi Mangione being glorified on social media? | Canada Tonight

The suspected killer of the UnitedHealthcare CEO, Luigi Mangione has been receiving a startling amount of online praise recently. David Gilbert, a reporter at Wired, discusses what he's seen online and why some might be glorifying Mangione.

'B.T.' never forgot humble roots, CEO says

Following a five-day manhunt, Luigi Mangione was charged with murder on Dec. 9 for the killing of Thompson in a brazen shooting outside a Manhattan hotel before an industry conference.

Mangione suffered from chronic back pain that impacted his daily life, according to friends and social media posts, though his specific treatment and coverage history was unclear.

"It's hard to underestimate the anger and angst people have with their insurance companies," said David Shapiro, a former FBI agent and professor at the John Jay College of Criminal Justice in New York.

Shapiro said he had never seen anything like the reaction to Mangione, but added, "it's not so far-fetched given the mood of the country and the ease of cheering this anonymously on the internet."

Several commentators on social media noted Mangione's privileged background as a member of a prominent Baltimore, Md., family, as compared to Thompson's working-class upbringing in rural Iowa, and said the murder was an example of how anti-capitalist rhetoric can incite violence.

Witty did not mention the shooting suspect, but he paid tribute to Thompson's humility, both in a letter to employees on Wednesday, and in his submission to the Times.

"His dad spent more than 40 years unloading trucks at grain elevators," wrote Witty. "B.T., as we knew him, worked farm jobs as a kid and fished at a gravel pit with his brother. He never forgot where he came from, because it was the needs of people who live in places like Jewell, Iowa, that he considered first in finding ways to improve care."