Your feelings aren't unusual. In general I'm against the death penalty. This opposition began because of miscarriages of justice, before I knew there was such a thing as the law being wrong I had nothing against executions. In the heat of the moment I do still want terrible people to die in great pain. That's a normal reaction for anyone, I think. In the end though, my deepest opposition is the same as how you feel now. I believe it damages the individual who must turn themselves into an executioner and since it's court sanctioned it makes us all complicit by allowing it. It seems like you got to understanding that several decades before I did.
If you ever need another reason to argue against it then the death penalty is not a good deterrent. Murderers don't work in that frame of mind. Also, juries are less likely to convict when the death penalty is at stake because they don't want to be complicit in taking a life, even a murder's life, that's why the British got rid of it.