Knapp v. Cardwell

finding constitutional Arizona’s subsequent judicial construction of the statute which allowed for consideration of any mitigating evidence | the use of collateral estoppel in criminal proceedings arises from the constitutional doctrine of double jeopardy, and its protection should not be afforded to the prosecution | ex post facto clause only prohibits detrimental substantive alterations of applicable law at time alleged offense was committed | where jury's reliance on sole aggravating circumstance was invalidated, state could not seek the death penalty despite possibility that evidence would support an aggravating factor not found or sought at first sentencing