Actually, it is established that people did evil *before* the Bore and knowledge of the Dark One. Semi tortured people for years before. They had criminals, though far fewer apparently. So the Dark One's continued existence is unnecessary for people to be able to know darkness.
No. Sealing away the DO removes his *physical effect on the pattern*. It does not remove the capacity for humans to choose to do bad things. *Destroying the DO* does remove the capacity for humans to choose bad things because there is no bad choice available to them to make.