Evil can be brilliant too.
Assume with me that the late Roman Empire was ruled more by the emporer than by the senate and that the emporer was, at best, evil in style. All was for and about his personal gain, yet they were smart enough to appease the masses with games and free bread. The ability to control and pay his armies led to his downfall, but centuries of cruel, selish rule was maintained before their fall.
Evil must be very intelligent indeed to survive. It must create a system were the choice is between two evils and the masses lack the wisdom to pick the lesser of the evils all the time. This will allow evil to flourish, with occasional setbacks to be sure, but never vanquished and destroyed, it remains inheirent in the system. Once enough are convinced the system is to big to fail, that it can never be abanoned at any cost, evil has found its refuge.
Secret police, exicutioners, torturers (information extractors if you prefer), petty tyrants, cruel nobles, black magic, dark arts, cults, religions, business organizations and guilds, mercenary armies: there are so very many places the practioners of evil can reside in plain sight and draw no deep concern.
Evil does not usually arrange to kill the fighter that took the magic sword it wanted. rather, it deploys envy and seeks the circumstance that will DESTROY the man and sword, because if it cannot have it, no one will. Evil will stand ready to fill power vacuums and place blame, create scape goats and wage war, always in the name of good, of course. Evil will create the threat if needed, like Nazi's creating border scirmishes to convince the public of a threat that doesn't exist, even if they have to dress their own citizens in uniform and murder them for bodies to display to a shocked public.
Evil wins because in order to not under estimate it, constant vigilance is required. That is hard to do, and if maintained long enough, evil will find a way to direct that vigilance towards its own gain.
Game of Thrones uses this ploy beautifully with the Watch at the Ice Wall. Time has diminished the vigil, evil rises again. Sabatours and enemy spies are on the prowl. Few in the south believe the legends of the Ice. The stage is set for the retun of Great Evil.
Evil will always work to undermine free mens liberty via limiting their percievable options. Force good men to choose between evils, and evil always wins.