It would be nice to make their lives a misery
Good enough reason all by itself.
but if cruel and unusual punishments (and the death penalty) 'worked', surely there would be no crime, no child abuse, no rape no murder etc.
I don't think you can eliminate such crimes. I offer a plausible method for cheaply reducing the numbers. Note that I also am dead keen on rehabilitation. I'd put TV's back in cells, (after the Daily Mail had them taken out). But I'd control the content. Lots of adult literacy, car maintenance, teach yourself plumbing that sort of thing. If you wanted porn or football you'd have to get good marks in class. Hotels have such controlled TV systems, buy one of them.
No civilised country has an effective death penatly. In America it's rare to be executed even if you're black, and it takes so long to go through the process, that people are claiming it's cruel to execute pensioners. Even when people do get executed, they are killed in a humane manner, and recently a supreme court ruled that they had to make lethal in injeciton nicer.
Even when Britain had it, execution was pretty rare.
In any case, I'm against the death penalty, It's not cruel enough, and the American experience is that it works out many times more expensive.
Most murders are in any case "heat of the moment" to some degree. Very few are planned, and of course unlike sex offences you only get to do it to a given person once.
It is not a coincidence that a large % of the murderers executed by the USA are mentally damaged.
For a deterrent to work, the person doing it must be scared. If they don't think they will be caught, or if they observe that the full punishment is unlikely then they give it much weight.
The murder rate in Britain dropped like a stone once there was a network of police forces. We can do a lot more to improve conviction rates, like investing in better forensics. Thus increasing the chances of being caught part of the equation.
Child abuse in families is not heat of the moment, and it seems to be the case that abusers do this many times. Given the absence of any idea on how to improve how often they are caught, the only lever I can see is very much more severe punishment. Braning on the forehead comes to mind, as not very painful, but allows for peados to be cared for in the community.
Note that most such "care" places are put in council estates, which have an on average higher % of kids per house. As noted in an earlier thread, local authorities are essentially bullies and see them as easier people to pick on.
in countries where these punishments exist? Which we all know is very, very far from the truth. That's why I feel I can say it doesn't work (which is what I was saying, rather than I know that you are wrong - it's really not personal), along with many deeper philosophical & moral arguments that I'm just not articulate enough to go into properly.