Standards for determining what constitutes "cruel and unusual punishment" were not defined explicitly in the Constitution, but set in the 1972 Supreme Court case, Furman v. Georgia. Wikipedia lists the criteria used to determine whether punishment is cruel and unusual, as specified in Furman v. Georgia:
The "essential predicate" is "that a punishment must not by its severity be degrading to human dignity," especially torture.
"A severe punishment that is obviously inflicted in wholly arbitrary fashion." (Furman v. Georgia temporarily suspended capital punishment for this reason.)
"A severe punishment that is clearly and totally rejected throughout society."
"A severe punishment that is patently unnecessary."
But, to clear up your confusion, the 9th Circuit Court did not find that the death penalty was inherently unconstitutional, but that capital punishment as currently implemented in California was unconstitutional. In California, the implementation of the death penalty is so plagued with delays and uncertainty, and takes so long that, if you are on death row in California, you are much less likely to die of execution than of other causes. Wikipedia lists the causes of death for California death row inmates since 1978, when capital punishment was reinstated in the state:
- 57 inmates have died from natural causes
- 6 inmates have died from other causes
- 20 inmates have committed suicide
- 13 have been executed in California
- 1 inmate (Kelvin Shelby Malone) was executed in Missouri
Because of this, 9th District judge Cornac J. Carney ruled that the California death penalty was implemented arbitrarily, and therefore unconstitutional under the 8th Amendment's prohibition on cruel and unusual punishment. From his decision:
[California's system of capital punishment,] where so many are sentenced to death but only a random few are actually executed, would offend the most fundamental of constitutional protections — that the government shall not be permitted to arbitrarily inflict the ultimate punishment of death.
As for the random few for whom execution does become a reality, they will have languished for so long on Death Row that their execution will serve no retributive or deterrent purpose and will be arbitrary
In other words, it is not that the cruel and unusual punishment is defined in such a way as to include the death penalty. (Many people and groups have tried to argue that it is, but those arguments have generally failed before the courts). Rather, it is that this judge ruled that cruel and unusual punishment is defined in such a way to include California's specific system of capital punishment.