Yet the ultimate problem is more fundamental. I have long believed that the death penalty is in all circumstances a barbaric and inhuman punishment that violates our Constitution. Even the most vile murderer does not release the state from its constitutional obligation to respect human dignity, for the state does not honor the victim by emulating the murderer who took his life. The fatal infirmity of capital punishment is that it treats members of the human race as non-humans, as objects to be toyed with and discarded.

Writing in Reason and Passion: Justice Brennan's Enduring Influence (1997).