One would think that demons (who hate God) and people (most of whom hate God) would have at least an amicable relationship. After all, they are both opposed to God, and are both living in open rebellion against him. Shouldn't a shared enemy unite them?
Yet going all the way back to the first satanically possessed serpent, fallen angles have a track record of hatred and violence against people. Satan propelled the world into sin, while demons afflict the weak and cripple the strong. Why? Why do demons hate people so much?
Part of the reason has to do with the fall-not Adam's fall, but Lucifer's. Obviously Satan fell because of pride, and because of his desire to be like God. But Scripture also hints at why, and it goes back to creation.
When God created the earth, it was good, beautiful, and perfect. Angels were part of creation. They did not exist before the first day, and one of their first acts of existence was applauding at God's work (Job 38:5-7). They were created, then they were immediately struck by the beauty and majesty of God's creation, the focal point of which is the earth.
Angels remained unfallen on day 6. After all, at the end of Day 6, God declared everything he had created as good. The universe could change (and it would), but for the time being all was perfect. Yet a question remained: who would have dominion on the earth? Who would rule the animals, and who would be God's regent over the planet?
The angels clearly appreciated creation. They applauded at it, and they were in awe of its beauty (Ezek 28:13). But God did not give the earth to angels. As Paul writes, "Now it was not to angels that God subjected the world" (Hebrews 2:5). Instead, God made man. He made Adam out of dirt, and Eve out of Adam. And then God gave them dominion of the earth instead (Gen 1:26).
This certainly would have been a shocking turn of events to the angels. Angels are spiritual, and people are literally made out of dirt. Angels are the sons of God, but people are in the image of God. Scripture expressly says that people are "a little lower than the angels" and that is certainly meant as an understatement (Psalm 8:5).
Why did God make people (even after angels)? Psalm 8:6 says, "You have given him dominion over the works of your hands; you have put all things under his feet." This is exactly what some of the angels wanted. At the very least, Satan wanted dominion over the earth. He wanted to be like God. In fact, he compared his beauty to the beauty of man and creation (Ezek 28:17). And you have to admit, he has a point. The obvious question is:

What is man, that you are mindful of him, or the son of man, that you care for him? You made him for a little while lower than the angels; you have crowned him with glory and honor, putting everything in subjection under his feet. (Hebrews 2:6-8)

Why would God do that? Why would he turn the earth over to people rather than to angels? For starters, we are in his image. Moreover, by making us frail ("frail children of dust, and feeble as frail" the hymn writer says) God is more glorified through our dependence on him. Because we are frail and sinful, God is glorified through our forgiveness of sins. The gospel comes through us, not through angels. This is why Hebrews 2 immediately follows the question about why people would receive dominion with this observation: it was fitting for Jesus to also be made lower than the angels (Hebrews 2:9).
Angels can't die. Yet the glory of God as seen in the gospel is bound up with the death of Jesus:

We see him who for a little while was made lower than the angels, namely Jesus, crowned with glory and honor because of the suffering of death, so that by the grace of God he might taste death for everyone. For it was fitting that he, for whom and by whom all things exist, in bringing many sons to glory, should make the founder of their salvation perfect through suffering.

In other words, God gave the earth to people (as opposed to angels) because the gospel comes through the death of Jesus, and his death is vicarious because of his perfection, which is demonstrated through his suffering. The end result is that many sons come to glory.
But this would not do for the devil. Deprived of his desired dominion, he was cast from heaven to earth (Ezek 28:16). And from that moment on, a cosmic war between Satan and people is played out on the earth. God's plan is to rescue people from sin through Jesus, and Satan's plan is to take out his jealous vengeance on people-the recipients of the earth. By opposing the gospel and afflicting people, he is seeking to undo God's design. By keeping people from the gospel attempting to mitigate God's glory, all the while exercising his rebellion against God by afflicting those to whom God gave the dominion that he felt should be his.
This is why Satan began is his attack in the garden. He attacked God's created order (by targeting Eve) and sought to undo their dominion. This is why angels cohabitated with people in Genesis 6, and this is why demons attack people. They have a desire for dominion, yet a hatred of us.
The result is that they afflict us as a form of rebelling against God's order. God commanded that angels serve people, and instead some angels as part of their revolt attack us, as their way of rebelling against the fact that we have been given dominion.