UnitedHealth Group CEO said Friday that U.S. health care system is flawed, but condemned the "vitriol" aimed at the insurance industry after the murder of UnitedHealthcare CEO Brian Thompson.

UnitedHealth Group CEO Andrew Witty wrote in an op-ed published Friday in the New York Times that system needs to work better. Witty said Thompson was working to make it better before he was shot and killed in New York on Dec. 4.

"We know the health system does not work as well as it should, and we understand people's frustrations with it," Witty wrote. "No one would design a system like the one we have. And no one did. It's a patchwork built over decades."

Witty said that while the system isn't perfect "every corner of it is filled with people who try to do their best for those they serve."

"We also are struggling to make sense of this unconscionable act and the vitriol that has been directed at our colleagues who have been barraged by threats," Witty wrote. "No employees – be they the people who answer customer calls or nurses who visit patients in their homes – should have to fear for their and their loved ones' safety."

He added: "Together with employers, governments and others who pay for care, we need to improve how we explain what insurance covers and how decisions are made. Behind each decision lies a comprehensive and continually updated body of clinical evidence focused on achieving the best health outcomes and ensuring patient safety."

The alleged shooter, 26-year-old Luigi Mangione, pleaded not guilty to murder and other felonies. He previously wrote a manifesto criticizing the health care system. Authorities said Mangione was not insured by UnitedHealthcare.