Justice Sandra Day O'Connor cast one of her most significant Supreme Court votes last week. And she came down on the liberal side, casting the deciding vote in a 5-to-4 ruling that struck down the death penalty for a 16-year-old cop-killer. The majority - O"Connor, Powell, Brennan, Marshall and Stevens - said the defendant should be spared the death penalty because of his "turbulant childhood." The four-man minority - Burger, White, Blackmun and Rehnquist - said it was time for the court to "bite the bullet" and stop intruding into the sentencing authority of state courts.