Taliban want to take control of the country make it some radical sharia state. Karzai wants to run Afghanistan and be western friendly. These two want to control the same country and have radically different ideologies which they want to impose onto the whole country. There is only 2 ways this can turn out 1) The Taliban win, chase out the NATO troops and overthrow Karzai 2) The Taliban loose and are either driven out the country for good or forced into surrender to democratic rule even if Karzai can't be described as a democratically elected president since he rigged the elections.