Demons suffer and get defeated in the end because they follow the wrong teachings. Demons get initial victory but this victory is short lived and is easily snatched away by Truth.
Demons consider dharma and moksha or liberation as empty dogma. They believe that the material world is the only reality.
They strive for victory, wealth, happiness, lust and longevity. They believe that everything in this world depends on the human body.
They believe that they are the children of night. This symbolically suggests the spiritual darkness of demons.
They consider devas, or demigods, who represent light as their arch enemies. They consider dharma and moksha and escape from birth and death as pointless and waste of time and energy.
Demons worship always for material gains. They use penance or Tapas to achieve their materialistic goals.
By following the wrong teachings, demons become embodiment of ego, desire, selfishness and lust.
Lust and anger are the two great challenges to virtue and the demons easily fall prey to it.