Sadness and darkness are natural cousins. Since ancient times, people have worn black to express their grief. Happiness is attracted to sunshine and depression to darkness. There is a certain romance in darkness and melancholy. There is something mysterious about that which is hidden and unknown. Dark poems may seek to romanticize sadness and depression. Other dark poems are simply poems about sad subjects. The poem "The Raven" by Edgar Allen Poe is an example of a delightfully spooky dark poem. There is much room for creativity in this genre.
Hatred is a poison that fills your body. It becomes impossible to think of anything else but the object of your hatred. Sometimes if you don't encounter the object of your hatred for a length of time, the hatred may dissipate throughout your body. You may be under the impression that the feeling is gone. The truth is that is has spread like a cancer. It is very important if possible to tell the person who you are angry with how you feel. If this is not possible it might be helpful to discuss your feelings with others. In any case do not let this hatred sit and poison your body.