I have heard (but do not know for sure) that the etymology of bella donna as coming from the Italian "beautiful lady" is not true, much like Marijuana doesn't really come from the Spanish Mary Jane.
One of the other possible explanations for the term belladonna is that it is a corrupted form of the Roman goddess of war, Bellona. The worship of Bellona was a rather secretive thing in ancient Rome, not officialy recognized. It involved people dancing around frenetically, while beating themselves with the flats of their swords, or possibly with the blades themselves. Such activity very well could have been fueled by taking the plant, which was after words called Bellona after the goddess. Belladonna could merely be a corruption of that.