The key to the decision is, of course, the determination of whether or not lethal injection is "cruel and unusual." Chief Justice John Roberts concludes the lead opinion of a 7-2 majority stating:
Throughout our history, whenever a method of execution has been challenged in this Court as cruel and unusual, the Court has rejected the challenge. Our society has nonetheless moved to more humane methods of carrying out capital punishment. The firing squad, hanging, the electric chair, and the gas chamber have each in turn given way to more humane methods, culminating in today's consensus on lethal injection. The broad framework of the Eighth Amendment has accommodated this progress toward more humane methods of execution, and our approval of a particular method in the past has not precluded legislatures from taking the steps they deem appropriate, in light of new developments, to ensure humane capital punishment.
Lethal injection is the most humane and least cruel method of execution that mankind has invented so far. It is also not unusual in that at least thirty states use the method for their executions. Lethal injection is by no means "cruel and unusual."
Chief Justice Roberts and the current court seems appropriately deferential to the will of the people as expressed through elected representatives, rather than appointed judges.
Capital punishment is, no doubt, controversial and subject to political debate. But that debate should properly be held in Congress and the legislatures of the states. The debate should not be undermined by backdoor legal attacks. Thanks to the Supreme Court for closing the back door.