I've seen a few documentaries showing death row inmates being lead to their fate, let's just say that even the hardest bad-ass in the cell block usually turns into a gibbering idiot when they know they only have minutes left to exist. To me, it's the fact that people often KNOW that even if they were to get the death penalty, they may well be pardoned, might be able to drag it out for decades, or that they might not make it in prison long enough to worry about something that could be years down the road. When you barely care about the current moment, considering that it could be 10-20 years before you could POSSIBLY be executed isn't much of a deterrent. Now, if such imates were to know that they'd end up in solitary confinement from the first moment they reached prison soil and that they had, say, 90 days before execution, I believe that would change the outlook of those who don't truly take the consequences of their actions as seriously as they need to.
I see the problem with the death penalty being "expensive" because far too many who are faced with it end up dragging appeals out for decades, adding greatly to costs. Let's face it - when you have someone like the Norway shooter who is caught in the act and could NOT blame the act on someone else, there wouldn't be any reason at all to allow appeal, simply drive him from the crime scene straight to where he'll meet his end and be done with it. I wouldn't condone an expedited execution of anyone who was NOT caught in the act of something that would be an offense worthy of such an end, but let's face it, the Norway shooter (for example again) is NOT going to be a productive member of society, he has zero chance of ever beginning to "make amends" for what he did, and an speedy execution is simply what would have happened to him were the victims' families allowed to make the call on what would be done with him.
... I've always felt that if more people who were on the verge of committing atrocities against others had actual FEAR of their own demise as retribution, some people might just reconsider their actions before making a terrible choice....
The problem is that studies haven't shown that to be true. It's also just as expensive to put someone to death as it is to keep them in prison for life, so I don't think economics is a useful argument either.
The problem is that it's damn hard to decide when someone can be rehabilitated and when they can't. I know I'm not qualified to make that judgement, and I don't think anyone else is either....