I don't know where people get the idea that Behemoth and Leviathan are supposed to be demons. In the Hebrew Bible and the Christian Old Testament, Behemoth and Leviathan are described simply as part of God's creation.
So are fallen angels...
Fair point. But in the Book of Job, Behemoth and Leviathan are simply described as creatures of might that will not be tamed by the hands of men (paraphrased, yes, but that is the essential gist). And later, in the Book of Isaiah, Leviathan's purpose is stated to be simply: "to frolic". Perhaps I'm missing a passage somewhere?
As for this artwork itself, the creature feels ominous and foreboding, and the coloring and shading do well to highlight this aspect. The glowing eyes and the small scale of the human underneath do well to emphasize the sense of power one might feel if one were confronted by a creature of this size.
Job well done.