I want to know why i need help....please..

Hello my name is Grey,

This is a question for someone who has physiatrist training or even studying to be one. My age is 20, I'm a white male. not sure if that will be needed but if so its there for extra info.

Recently I have found myself asking alot of question to why I'm depressed or why I have began to think of violent things, or even why I feel the need to seek help. I need to know why? I know that something is making me feel this way I just can understand it, and it is starting to majorly affect my life.

For information:

A little history for an insight into my life if it helps. I grew up in an average neighborhood with a paraplegic father, who passed away when I was 7. Up until 5th grade I was a straight A student, after that I rapidly found my self starting to care less and less about issues that a "normal" person would have found alarming. Such as a major decline in grades from A's to D's and F's. Graduated high school and shortly after i turned 18 I moved out on my own, where I have been supporting myself for the past two years. At the age of 19 I met the love of my life, and my now fiance whom i plan to wed next year.

Only this year has it became exceedingly apparent to me that my depression is winning the battle within me. I want to figure out why so i can figure out why this is happening so i can get my life back in order. The depression or what ever the true diagnosis is, has led me to have thoughts of robbery, and other things of the sort. As well as caused me to become a much meaner person than i was before. To anybody out there that even thinks they can help I would gladly answer any questions if you feel it might help you to help me understand this.