In a New York Times op-ed published Friday morning, UnitedHealth Group’s CEO addressed boiling frustrations, threats and vitriol aimed at health care insurers, conceding, “We know the health system does not work as well as it should.”

But he said slain executive Brian Thompson was one of the people trying to make that system better.

Andrew Witty, the head of UnitedHealth Group, the parent company of United Healthcare, wrote that he understands “people’s frustrations” with the American 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. Our mission is to help make it work better,” Witty said in his op-ed, titled “The Health Care System Is Flawed. Let’s Fix It.”

Witty described Thompson, the CEO of UnitedHealthcare, as a champion of that mission. “As Brian Thompson’s family, friends and colleagues mourn his killing, we are bearing a grief and sadness we will carry for the rest of our lives. Grief for the family he leaves behind. And grief for a brilliant, kind man who was working to make healthcare better for everyone,” Witty wrote.