UnitedHealth Group CEO Andrew Witty says the healthcare system is flawed and United's mission is to help improve it.Kent Nishimura/Getty ImagesUnitedHealth Group's CEO eulogized his slain colleague, Brian Thompson, in a New York Times op-ed.Thompson's shooting sparked a nationwide debate about the state of the health insurance industry.Witty's op-ed faced fierce online criticism from people who said it didn't offer solutions.The head of UnitedHealth Group, the parent company of UnitedHealthcare, is responding to the "vitriol" that's been lodged — both on- and offline — against the health insurance industry and its workers.In an op-ed published in The New York Times on Friday, UnitedHealth Group CEO Andrew Witty expressed his grief over the fatal shooting of UnitedHealthcare CEO Brian Thompson last week, as well as his appreciation for the "outpouring of support" for Thompson.