Two execution disputes the U.S. Supreme Court will hear next term with swing vote Anthony Kennedy off the bench could foreshadow its treatment of certain death penalty cases going forward.

Kennedy didn’t join efforts to strike down the ultimate punishment completely, but he was a swing vote when it came to its application. He helped form coalitions banning executions for more vulnerable offenders but also ones condoning controversial execution practices.

His exit brings into sharp relief the relative lack of...