Demons are a different sort of being than a Devil. Although sometimes difficult to tell apart (They're all evil, dangerous, and often look very much alike) they are completely separate types of entities.
Where a Devil is keen on rules and contracts and are exceptionally proud a demon is far more loose in its ways.
Demons are more manifestations of emotions without restraint than anything else. They are wild, chaotic and depending on their magnitude more or less difficult to bind. Eventually any bound demon, because of their entropic nature, will break free.
If this happens, before the demon in question has been sent back to wherever it came from, pretty much anything can happen.
Demons are, as a rule, less powerful than Devils but also easier to use and control because they must not be bound by carefully phrased contracts or deals, but can be kept in check with brute-force spells of binding.