Dystheism is the idea that there is a single omnipotent God alright, but He doesn't work quite right. It is the belief that God is dysfunctional -- and if we are God's children, we're children in a dysfunctional family. Logic does not require that any Creator-God must be good, or even sane, and much that goes on in the Universe could be very well explained by a wacky and amoral higher power, tossing out whims with no concerns about the consequences (and, naturally, for that being the consequences would be none).
One of the earliest reason-based concepts of God, the Emanationism of the ancient Greeks, fit into dystheism in its essential character. It envisioned a God of such power that the Universe emanates from it with no effort or desire; we exist not because God chooses to create, but because Creation oozes from God both thoughtlessly and effortlessly.
Dystheism is easily confused with misotheism, hatred of God even against a God that is good. But the dystheist can still (somewhat oddly) love God; and the theist who mistakenly believed a dystheistic God to be good can love God in ignorance of His true wackiness. It may also be confused with the tendency of polytheism-based faiths to have a bad God or two in their decks -- although, attention reveals that the dystheistic God is not evil, for that would be an example of maltheism. The Dystheistic God simply doesn't care, or lacks either good or evil.