Daleks were evil, wanting only to destroy anything that wasn’t a Dalek and expand their empire. They thought themselves so pure and perfect that everything else in the universe was inferior and therefore could not be tolerated.
The Doctor had been battling them throughout time and space ever since their creation, but no matter how often he defeated them they always came back, stronger and more formidable than ever. Despite their certainty about their own perfection, they were still evolving, adapting, reaching towards their ultimate destiny.
But the Doctor had a destiny too; to stop them any way he could.