I would say almost all of my dreams employ dark, sometimes depressing or cruel themes. So I can say that that is normal. However, I have a creatively inclined mind with a love of all things dark and scary, so that comes naturally to me. It's the reason I love having nightmares (most of the time) because they provide excellent material for my creative works and after a while my mind just began to spew these assembles of dark images. If you read my dream journal you'll see that there's always some kind of dark theme. Although on a rare occasion I do receive a serene or happy dream, usually a love dream or some sort of pleasurable alternate life. So no, you're not alone.