ispar, you've hit the nail.
Claiming that human life is so sacred that it must be revenged by taking another life is a paradox argument.
As a consequence? Well, it's the society that dictates the consquenses and the society has nothing to gain from executions. If the society should start to execute those citizens that are a 'burden to the economy' we will end up in a 'Orwellian' or 'Fascistic' kind of state, where only the healthy and strong have the right to live.
The human life vs life in general is an age old debate, and there are sevral philosophical directions to take. Arguing that a human life is worth more than a life of dog is not nessecarily hypocracy. It's merely a choice of philosophical stance.
One main argument against death penalty to me is that death can never be undone. Death is final, and as far as we know (can know) there's no 'after-life', no 'nirvana' or anything like that.
And with an imperfect justice system we're bound to deny innocent people their lives, which is utterly and outmost horrible.
I know this is streaching the point a bit hard, but to make a point and to really understand what you are discussing you sometimes have to extend arguments to its extremes.
There are ONE valid (IMO) argument for death penaly, and that is revenge, revenge on the behalf of the relatives of the victims, and I can actually appreciate this argument. I can understand the pain and agony that calls out for revenge, I know I'd crave revenge if put in that position.
It's not an easy judgement to make, but I hope I've explained my view on the matter.