In Christian theology, "demon" is a category of beings, the set of angels who rebelled against God.
"Devil" is one specific individual, the leader of the demons.
There is only one devil but there are many demons.
"Devil" is sometimes used to refer to any evil person, as in, "Joe is a devil".
But I think that's a metaphor.
Like people will say of a smart person, "Wow, Bob is a real Einstein".
ell.stackexchange.com/users/803/Jay
Demon and devil can be used as synonyms to refer to a wicked person.
So I think these words can be used interchangeably in the same sense as the sentence presented by the OP.
In this sense, both the words can be used in the singular and plural, for example, "Be careful - he is a demon/a devil (not the devil) or they are demons/devils".
ell.stackexchange.com/users/9259/Khan
I would say that, using them toward a person would be basically equivalent, referring to evil, and would probably be a metaphor or simile depending on context.
However, in theology or fiction there are several different connotations
Demon A demon is any of several types of evil creatures from another plane of existence.
Most frequently, red or purple or black.
Devil
The Devil (with the definite article), refers to a specific entity.
(see Wikipedia .
Usually presented as a red humanoid with horns, or sometimes appearing as a human, in either form, wings could be present.
In its plural, it refers to any of the servants of the Devil with similar appearances.
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There is a subtler, connotations based, difference that neither of the answers have touched on so far.
A Devil is strongly associated with religion and sin.
In almost all contexts a devil, or the devil, represents evil.
Although it may sometimes be light-hearted; "You devil!
" could be playful, depending on tone of voice.
A demon does still appear in religious contexts but is also found in other references to the supernatural.
Although they can be monsters, demons are sometimes not portrayed so negatively.
They can be romanticised, for instance "
Sweeney Todd; the demon barber of fleet street.
" or in Kubla Khan "By woman wailing for her demon-lover!".
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Demon and devil come from theological terms and still retain those meanings.
A wicked person could be called a demon and be meant to be a supernatural demon, or said to act devilishly and meant to be literally "like the Devil."
The other answers provide great background on those.
In modern non-religious usage: Devil/devilish can mean mischevious (like a sneaky child), or something that strongly appeals to the senses and would be tempting.
A food or woman could be described as devilish .
Devilish can mean evil, but usually diabolical takes that role.
Demon is associated with the concept of obsession ("he studied every hour of every day like a demon.")
doing something so well (or other quality)
that one might not consider the person doing it human.
Can imply obsessive or obsessive-like dedication to the skill.
First thing that comes to my mind is speed demon .
ell.stackexchange.com/users/5144/LawrenceC
The devil has a unique shape, derived from the Greek god Pan, the god of shepherds and flocks.
He had the attributes of a goat, horns, legs and tail of a goat.
The Christian religion took over the figure and made him the epitome of everything evil and the oppponent of God, which the Greek Pan was not at all.
A demon is a bad spirit who can have any shape.
Demons can even be invisible and they occur mostly in folklore.
ell.stackexchange.com/users/4159/rogermue
As far as I recognize, people use "demon" as a kind and can be used in plural form yet devil is not being used in plural.   He is a demon, stay away from him!  He is the devil, stay away from him!   So what's the difference? In which situations I use demon and devil?
The difference between "devil" and "demon"
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<meaning><word-usage><word-choice>
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ell.stackexchange.com/users/28345/Cenkay V.
Demons are bad spirits, evil creatures from another plane of existence. They may  be invisible and they occur mostly in folklore. In Christian theology, "demon" is a category of beings, the set of angels who rebelled against God. Demon and devil can be used as synonyms to refer to a wicked person.  "The Devil" is one specific individual, the leader of the demons and it represents evil. There is only one devil but there are many demons.  Demons are bad spirits in theology. The devil is their leader. Both terms can be used to describe wicked people.
Demons are bad spirits in theology. Both terms can refer to wicked people. The devil is the leader of demons.