I believe because many emotions and thoughts (not all) are dark forces (demoniac forces) at work that people yield to.
I don't believe in Manichaeism, the view that there's a devil and God in equal opposition to each other. That is also a Christian heresy not that I subscribe to Christian theology but in this instance my perspective and orthodox Christian theology agree.
From my perspective there are lower astral entitles that seek to experience vicarious pleasure in people's negative deeds. Those lower astral entitles are often labeled as "demons".
In the larger sphere, I accept the Indian concept of "maya", illusion that automatically seeks to promote ignorance but is really an aspect of the divine doing divinely created work. On the human level maya appears as ignorance which creates the illusion that we're really separate from Divinity.
Maya is necessary to create the adventure of life because by overcoming maya we realize our true natures. This is like the mountain that can be climbed or the obstacle course in running.