To be at the bottom of something: to be the real cause of something. Example: The desire for money is at the bottom of much of the violence in the world.
bottom of the barrel and bottom of the heap
Fig. the location of persons or things of the very lowest quality; someone or something of the lowest quality. (The fruit at the bottom of a barrel of apples is likely to be bruised from the weight of the other apples.) That last secretary you sent me was really the bottom of the barrel.