The Washington Supreme Court has found the death penalty to be unconstitutional because it violates the state constitution, specifically Article 1, Section 14, which states, “[e]xcessive bail shall not be required, excessive fines imposed, nor cruel punishment inflicted.” From the opinion in State v. Gregory, No. 88086-7 (Wash. Oct. 11, 2018):
“The death penalty is invalid because it is imposed in an arbitrary and racially biased manner. While this particular case provides an opportunity to specifically address racial disproportionality, the underlying issues that underpin our holding are rooted in the arbitrary manner in which the death penalty is generally administered. As noted by appellant, the use of the death penalty is unequally applied—sometimes by where the crime took place, or the county of residence, or the available budgetary resources at any given point in time, or the race of the defendant. The death penalty, as administered in our state, fails to serve any legitimate penological goal; thus, it violates article I, section 14 of our state constitution.”
For an analysis, see this blog post by Reba Kennedy.