I like keeping a diary. I never realized, but I'm so angry in my posts. Does this mean that I am depressed?
Not necessarily. Keeping a diary, even one with angry posts, is a great way to express your feelings in a healthy, controlled manner. It can be a cathartic experience and, as you said, you enjoy doing it. If you lose interest in something you enjoy (like writing), notice a change in your eating or sleeping habits, or have feelings of worthlessness, then consider talking to your doctor about possible depression.
Not necessarily. You may just be very angry; perhaps there are patterns in this anger that cause you suffering? It's healthy that you notice this going on inside you, so you can deal with it. Anger may show you areas of life or relationships where you feel taken advantage of or need to address something. Talking with a therapist about it could help you gain clarity before acting on any of it, if you choose to.