The slippery slope begins far, far before today's decision. It was when execution was decided to be a fit means of punishment. If you're consistent on believing that is wrong, then at least you're making a consistent argument. But if you want to get the boulder back in place, you've got a long, long ways to shove.
It was when execution was decided to be a fit means of punishment. Capital pushishment, and human government's right to exercise it, has been a part of Judeo-Christian civilization, and specifically a part of American governance, all along.
Punish the guilty, protect innocent human life. The former establishes respect for the latter. It's not complicated.