The accused killer of UnitedHealthcare CEO Brian Thompson was carrying a three-page manifesto when he was captured Monday afternoon at a McDonald’s in Altoona, Pennsylvania. An Ivy League graduate with a masters degree in engineering from the University of Pennsylvania, Luigi Mangione, 26, suffered from crippling back pain for which he had undergone spinal surgery. According to The New York Times, his manifesto included the lines, “These parasites had it coming,” and, “I do apologize for any strife and trauma, but it had to be done.” (Mangione’s lawyer has said his client plans to plead not guilty to charges including second-degree murder, forgery, and three gun-related offenses.)

While no one can seriously justify the decision to gun down a father of two to make a political point, Americans seem to agree on one thing: Something does need to be done about the country’s broken health insurance system.

The brutal slaying, and the frenzied manhunt that followed, have exposed widespread frustration with a system of for-profit health insurance that many Americans feel is actually killing them, one delayed-or-denied health insurance claim at a time. And doctors are as fed up as their patients.

“It is robustly obvious that there needs to be massive health care reform,” says a Texas emergency physician involved in health policy, whose hospital system will no longer let her speak on the record about these issues. “It is a universal truth now that patients in America are angry, that physicians are angry, and that commercial insurance isn’t in the health care business. It’s in the financial business.”

Struggling with the seemingly arbitrary policies and undermarket reimbursements offered by UnitedHealth Group and other insurers, doctors feel trapped in a zero-sum game that has robbed them of their clinical independence.

Four companies, known collectively as BUCA—Blue Cross and Blue Shield, UnitedHealth, Cigna, and Aetna—have become so powerful that “when doctors try to negotiate, they have to take whatever that carrier gives them,” says Ron Howrigon, a health care consultant who represents doctors in their dealings with insurers. He says of Thompson’s killing, “Obviously, this is a tragic event. But one of my first thoughts was, I am kind of surprised this hasn’t happened earlier.”

Amid this anger, UnitedHealth stands out for its sheer size, its aggressive moves to vertically integrate healthcare, and its expansive denial of claims, say health care experts and doctors. Headquartered in Minnetonka, Minnesota, it is a behemoth that took in more than $370 billion last year, making it America’s fourth-largest corporation by revenue. It insures more than 26 million Americans. More than one tenth of US doctors are either employed by or affiliated with the UnitedHealth subsidiary Optum Health, according to numbers shared by a company official at an investors conference last year. According to data from the personal finance website ValuePenguin, UnitedHealth denies almost one third of the patient claims it receives.

An investigative series by the biomedical news outlet STAT has likened the company to a “modern-day Standard Oil” that has built a profit-making machine by sitting on both sides of the transaction: It insures the patients and also employs the doctors who treat them. It reportedly pushes its doctors to report unnecessary or exaggerated insurance codes, increasing reimbursement in part by fabricating maladies. (A UnitedHealth spokesman denies those claims, saying the company’s doctors make “independent clinical decisions” in compliance with federal guidelines.)

Journalistic reports, followed by a class action lawsuit, have revealed the company’s use of artificial intelligence to deny claims for elderly patients in its private Medicare Advantage plans. The company is also facing an ongoing Justice Department antitrust probe examining the acquisition of doctors’ practices by a UnitedHealth subsidiary.

A UnitedHealth representative tells Vanity Fair that allegations of monopolistic practices and aggressive claim denials are false and misrepresent the group’s core mission: to reduce rising health care costs for consumers and employers. “That’s a responsibility we take very seriously,” the representative says, adding, “We do not use AI to make coverage determinations.” The representative declined to comment on the Justice Department probe.

However, he says of coverage decisions, “Less than 1% of claims are denied for clinical reasons. There is a very big difference between a claim denial and a care denial. One is denying care and one is denying a claim. I don’t want anyone to think we don’t provide care, because that’s different.”

Doctors in independent practice groups describe strong-arm tactics by the company—suddenly pushing them out of network, slashing their reimbursements, and stranding their patients with almost no notice. “Their interest is in killing private practice physicians,” says a North Carolina anesthesiologist. “It’s all a commodity to them. ‘If we can pay you 20 cents on the dollar, we are going to do this.’” He adds, “It’s going to cause [physician] groups to collapse.”

The travails of Rheumatology Associates, P.C., the largest private rheumatology practice in Indiana, is a case in point. UnitedHealth offered the doctors in the group such paltry reimbursements that negotiations on a new contract ground to a halt, says Tony Drykos, Rheumatology Associates’ operations director. When UnitedHealth refused to extend contract negotiations, 1,600 of the practice’s patients were left without covered rheumatology care.

The group met with the Indiana attorney general’s office and also wrote a searing three-page letter to its patients, explaining:

Over the last 20 years, our practice has seen United Healthcare shape health care in a way that has allowed them to have full control over every aspect of your health care from having their own pharmacy benefit manager to infusion centers. This is called “vertical integration”, and it is a dark force in driving up costs and leading to worse health outcomes.

A UnitedHealth representative says that the rheumatology practice was demanding a price hike that would have made the group “significantly more expensive.” He says, “We remain open to discussions should they provide a proposal that’s affordable for consumers and providers.”

Drykos says that the company’s conduct was often nightmarish, even for patients in network. “Even when we get approvals on their letterhead, they would turn around, deny claims, and say they were approved incorrectly.”

He adds, “They have so much power, they are out of control. They believe they are untouchable.”

With Mangione facing prosecution, and Thompson having been laid to rest at a private funeral, UnitedHealth’s customers are left to wonder what remedies, if any, will be available to them.

The incoming Donald Trump administration is more likely to defang the Justice Department’s antitrust efforts, and less likely to rein in corporate abuses.

Trump himself, who spent almost a decade trying to dismantle the Affordable Care Act, which provides health insurance to 45 million Americans and prohibits insurance companies from denying coverage based on preexisting conditions, does not seem to have any platform to address these issues.

When asked in his September debate with Vice President Kamala Harris how he would replace or reform the act known as Obamacare, he said only that he had “concepts of a plan.”

His nominee for health and human services secretary, Robert F. Kennedy Jr., a known vaccine skeptic, has pledged to get to the bottom of what’s driving the problem of chronic disease in America. It is not clear whether, if confirmed, he would seek to reform the health insurance marketplace, and he did not respond to a request for comment. However, any patient who’s gotten a claim denial from their health insurer will have their own thoughts about how to “Make America Healthy Again,” the preferred slogan for Kennedy’s reform efforts.