The universe is a bit more complex than you are thinking.
The following is just a fraction of it.

All "demons" that I have seen were lost spirits, sometimes called evil spirits. The strength of these types of spirits comes from the herds of spirits that are connected to it.
These are usually follower spirits and victim spirits. There can be 100s of them, which makes it seems that one "evil spirit" can be as strong as 100 people. They can give "irresistible" thoughts or feelings to some people. Or give certain people a psychosis. Like: "You like little children don't you?" with a feeling of intense guilt. (I talked to a victim)

The "evil spirit" itself is broken in pieces. Each with their own mind. It is like 5 people in a psychosis. Nobody knows who is in control. And people in a wild psychosis can also act "as if they are possessed by the devil". Or think that it is completely ok to smash a stick on someone's head.

The broken mind comes from a mixture of several severe traumas. These always include the trauma of painful death and suffering. There can be murder or rape or starvation, or whatever.

These spirits have the same nightmare over and over again. Because these spirits are dead they are incapable of making any other choice. So they can not step out of it. In that sense, they are in hell. It does not matter whether they were evil or not. The follower and victim spirits are also in this hell, playing a different role.

To escape this endless suffering and trauma, these spirits try to disturb other people's lives. By doing that they escape their nightmare. But also do they bring their nightmare to the living people. Usually in dreams, but sometimes in actions and events.

So in this sense "demons" do not exist. But bad spirits do exist. My opinion is that every spirit is bad, until it has moved on to the afterlife. Many spiritual people think that there are good spirits that are still here trying to help. But I see that every staying spirit is a parasite on living people. It steals that life-energy from us. And because it is stuck in a dream-world, it can never help us with our real world. It is stuck in a dream-world, usually because it has some kind of trauma too.

And these bad spirits can be helped in different ways. For some people it helps to meditate or make connection with the afterlife. For some it works to pray, but that can also attract weird religious spirits. Also giving them "love" can attract weird spirits. But all this is not too bad if you are careful.

Hating these spirits does not help, it even attracts them. I sometimes encounter people that have become psychotic, because they kept of fighting these spirits. Giving them energy and reason to stay.

The simplest thing is that nature can help too. Nature automatically helps spirits to the next phase. Nature is always connected to the afterlife. Animal spirits do exist, but usually are they victim-spirits held by human spirits.