No. You're not stealing from anyone. It's decentralized and bits have no owner.
If you say 'hey, here is my Bitcoin transaction' and others accept, and you just had luck to find a valid address, it is not the same as stealing. Bitcoin has no value.
That isn't true, although you might like the philosophical debate around that, a judge is still going to make you pay the person back and/or give you a prison sentence if it's proven that you are the original owner of the keys. He's going to laugh at your digital philosophy as he slams his hammer down upon judgement.
Not if the bank scenario holds. There is no legal persona associated with bitcoin nor the block chain. There's also no money there. Someone else took it already.
