It seems inevitable that if it's possible it will be done.Ant:
Why SHOULD we? Just because we can? Does that line of reasoning immediately follow our scientific prowess - we can so therefore we ought to.
I think we are flat out just curious to see if it can be done. For one, there's the prosepect of better understanding our own brains and how they function. What is and isn't possible. Then there's the idea of "the other". If the universe were absolutely littered with intelligent aliens the distances and time constraints might still mean our best chance of contacting another kind of "person" might still be to create AI on earth.
Then there's preservation of human minds. A sufficiently powerful computer which mimics the activity of a human brain to perfection would give the illusion of immortality for passed loved ones. An integration with mechanical components in life might lead to literal immortality...
http://www.booktalk.org/the-infinite-human-t11540.html
I think what we fear the most is that a new computer AI that was at our level of ability would shortly be able to invent a better computer that was far beyond our abilities which would quickly ramp up to god-like levels of power. An almost all-knowing intellect whose life was not tied to the existence of any physical manifestation which would be able to exert control of almost any aspect of modern life.
Not finding a god waiting for us when we became self aware, we might make one ourselves. We worry that this one might be as capriciaous and angry as our ancestors imagined. But maybe it wouldn't need to hate us to be the end of us. Maybe just not caring about us would be enough.
Fun thoughts!
As to accountability... An AI on a murder spree or a human might have essentially the same culpability. You can say for instance that people who go on shooting rampages usually have some kind of hideous event in their past, or a systematic pattern of abuse that left them broken and ostricised from humanity. They didn't create the situation that led to them being murderers, but all the same they were the ones pulling the trigger.
So imagine you are in a factory that deals in molten lead. one day a valve opens up above the cafeteria and people die. It isn't the valve's fault that it killed people, but it still has to be stopped or it might open again. You'll need to trace back all the engineering mistakes that lead to that valve being placed in such a dangerous place, find the system of incompetence that leads to dangerous valves being placed above people's heads to REALLY fix the problem once and for all... but in the mean time, that valve has to be stopped.