Except for the one about 'no doubt whatsoever', I don't buy your conditions, however, despite having supported the death penalty for most of my life, I now object to it for several other reasons.
One, there have been too many (it doesn't take very many to be TOO many) people on death row found to be innocent when modern testing was used, and too many times, it was found that the cops were in a hurry to solve and crime and didn't care how they did it.
Two, it isn't sure enough or fast enough to be a deterrent. Potential criminals figure they aren't going to be caught, if they are, they won't be convicted, and if they are, they won't be sentenced to death, and if they are, it'll take 20+ years.
I too am against the death penalty for many reasons. One being it's ineffective deterrent. Another being, if I knew beyond a shadow of a doubt, that someone had killed a family member, with no provocation, just for money or a thrill, then I would kill them myself. Without remorse.