On Tuesday, the Supreme Court refused to review the constitutionality of Thomas Arthur’s impending death by lethal injection, effectively clearing the way for Alabama to execute him. Only two justices, Sonia Sotomayor and Stephen Breyer, would’ve considered Arthur’s constitutional claims. To emphasize her disgust with the court’s nondecision, Sotomayor penned a dissent, joined by Breyer, explaining why Alabama’s treatment of Arthur likely violates the Constitution. Her mordant opinion reaffirms her deep skepticism of lethal injection’s legality—and cements her position as the court’s chief critic of state efforts to wriggle around the Constitution and inflict punishments that are almost certainly “cruel and unusual.”

Arthur’s problems are intertwined with the difficulties that have plagued lethal injection for years. In 2008, the Supreme Court held that a lethal injection protocol involving sodium thiopental, a barbiturate that rendered the prisoner unconscious, did not violate the Eighth Amendment’s ban on “cruel and unusual punishments.” The next year, however, the only American manufacturer of the drug suspended its domestic production. States switched to a similar barbiturate, pentobarbital, but that drug became unavailable in 2013. So states began using midazolam as the first drug in the protocol, designed to render the inmate unconscious before the second and third drugs paralyze him and stop his heart.

Unfortunately, midazolam doesn’t induce unconsciousness, at least not consistently. While it has some anesthetic effect, midazolam cannot always maintain unconsciousness and seems to have a “ceiling effect” whereby increasing the dose will not result in greater effect. Multiple inmates have awoken during their executions, apparently because midazolam stopped working, causing them to feel unimaginable pain. Yet in 2015’s Glossip v. Gross, the Supreme Court rejected claims that the use of midazolam violates prisoners’ constitutional rights. The conservative majority wrote that to mount a successful Eighth Amendment challenge, an inmate must meet what Sotomayor’s new dissent calls a “macabre challenge”: He “must not only prove that the State’s chosen method risks severe pain, but must also propose a ‘known and available’ alternative method for his own execution.”

That’s a high bar—but Thomas Arthur appears to have met it. In district court, Arthur put forth an immense amount of evidence that midazolam creates a risk of “serious illness or needless suffering” for all inmates and specifically for him. Arthur has heart disease, and in the expert opinion of a doctor who testified on his behalf, midazolam would probably induce a “painful heart attack” while Arthur remained conscious. He then proposed a “known and available” alternative: the firing squad.

“Some might find this choice regressive,” Sotomayor writes, “but the available evidence suggests that a competently performed shooting may cause nearly instant death. In addition to being near instant, death by shooting may also be comparatively painless. And historically, the firing squad has yielded significantly fewer botched executions.”

But the district court judge ruled that midazolam didn’t really pose a heightened risk to Arthur and found that execution by firing squad was not “known and available” in Alabama. The U.S. Court of Appeals for the 11th Circuit affirmed that decision, declaring that firing squads aren’t authorized by statute. Now that ruling will stand, and Arthur will be put to death in a method that may very well cause him extraordinary physical agony.

That doesn’t sit well with Sotomayor, who rang the alarm on midazolam in 2015—calling it “the chemical equivalent of being burned at the stake”—and continues to ring it with admirable urgency. First, Sotomayor explains that firing squads may be available in Alabama, despite the state’s assertion to the contrary. State law, after all, provides that “if electrocution or lethal injection is held to be unconstitutional … all persons sentenced to death for a capital crime shall be executed by any constitutional method of execution.” This statute would seem to mean that if lethal injection cannot be constitutionally administered, and electrocution is unavailable, the state is empowered to consider alternative methods, including the firing squad—which, Sotomayor notes, is legal in Oklahoma.

But Sotomayor also points out a more serious flaw in the 11th Circuit’s logic, even accepting Alabama’s assertion that it cannot set up a firing squad: The decision would permit a state, “by statute, to bar a death-row inmate from vindicating a right guaranteed by the Eighth Amendment.” As she explains:

Under this view, even if a prisoner can prove that the State plans to kill him in an intolerably cruel manner, and even if he can prove that there is a feasible alternative, all a State has to do to execute him through an unconstitutional method is to pass a statute declining to authorize any alternative method. This cannot be right.

“By conditioning federal constitutional rights on the operation of state statutes,” Sotomayor continues, the 11th Circuit decision “contravenes basic constitutional principles.” The court, after all, has said that prisoners are entitled to relief when they prove that a method of execution is needlessly cruel and a known alternative exists. But Alabama has worked around this command by discontinuing all forms of execution except the one that is needlessly cruel. The 11th Circuit found that because no alternatives exist, the state can move forward with a needlessly cruel execution. Under this view, Sotomayor notes, an inmate’s ability to avoid an “intolerably cruel” execution “depends not on the Constitution but on vagaries of state law.”

Sotomayor concludes with a stinging observation: The 11th Circuit decision gives states a road map by which to short-circuit the “evolving standards of decency” that guide Eighth Amendment jurisprudence. “These evolving standards have yielded a familiar cycle,” Sotomayor writes:

States develop a method of execution, which is generally accepted for a time. Science then reveals that—unknown to the previous generation—the States’ chosen method of execution causes unconstitutional levels of suffering. A new method of execution is devised, and the dialogue continues. The Eighth Amendment requires this conversation. States should not be permitted to silence it by statute.

In a trenchant passage, Sotomayor details the grotesqueries of hanging—including “slow asphyxiation and violent decapitation”—as well as the “ghastly” history of the electric chair, which persisted “until the specter of charred and grossly disfigured bodies proved too much for the public, and the courts, to bear.” The states “then tried lethal gas,” which “exacted ‘exquisitely painful’ sensations of ‘anxiety, panic, [and] terror,’ leading courts to declare it unconstitutional.” At that point, most of them switched to lethal injection.

But “science and experience,” Sotomayor writes, “are now revealing that, at least with respect to midazolam-centered protocols, prisoners executed by lethal injection are suffering horrifying deaths beneath a ‘medically sterile aura of peace.’ … What cruel irony that the method that appears most humane may turn out to be our most cruel experiment yet.” She then details several recent botched executions and proclaims: “Like a hangman’s poorly tied noose or a malfunctioning electric chair, midazolam might render our latest method of execution too much for our conscience—and the Constitution—to bear.” Here is her striking peroration:

Twice in recent years, this Court has observed that it “has never invalidated a State’s chosen procedure for carrying out a sentence of death as the infliction of cruel and unusual punishment.” In Glossip, the majority opinion remarked that the Court “did not retreat” from this nonintervention strategy even after Louisiana strapped a 17-year-old boy to its electric chair and, having failed to kill him the first time, argued for a second try—which this Court permitted. We should not be proud of this history. Nor should we rely on it to excuse our current inaction.

On capital punishment, the court is clearly split 4–4. Breyer, along with Justice Ruth Bader Ginsburg, believes it is unconstitutional; Sotomayor, along with Kagan, believes it is currently being inflicted in an unconstitutional manner. Meanwhile, the four remaining conservative do not seem to mind agonizing executions—and if Neil Gorsuch is confirmed, he will surely provide a reliable fifth vote to uphold both specific executions and the death penalty more broadly. For Sotomayor, this may be a losing battle. But it is one she is committed to fighting, without tolerance for the rationalizations that some courts use to euphemize the grisly business of state-sponsored killing.