Well, we have to think about the consequences. Will anything good actually come of it? Clearly the victim (of capital punishment) cannot be rehabilitated and become a functioning member of society anymore. However, they won't be able to hurt anyone else ever again either, which has to be a good thing. But they're unlikely to be able hurt anyone again if they are simply imprisoned, too. Not only does imprisonment have the added benefit of being cheaper (for a whole host of complex reasons that I suspect are insurmountable), but it also doesn't 'brutalise' the population as the literature seems to indicate capital punishment does.
I can't really see any good reasons for it at all. A popular one seems to be that some criminals just 'deserve' it, but this seems to be a very difficult position to justify for anyone who is at all concerned with the suffering of other people.