Please check my essay?

Okay, so the topic is ''The End Of Innocence'' and it's a narrative essay. I wrote on the Far Cry 3 game.. Anyway, please help me correct it. My teacher said that it needs to be more credible as well. All help would be appreciated!

Here it is:

They said that I could be whatever I want, so I became a hero. People did not see it this way because it changed me, but I did because I knew why I did what I had to do. It was the only way. I had my revenge and it was the end of my innocence but I became someone else in the process. Life sometimes forced the unexplainable feelings to be tasted by those whom are innocent.

I was born and raised in California, alongside my two brothers, Grant Brody and Riley Brody. Throughout my youth I have received admirable grades in school and I became a proficient college athlete.

My dad passed away shorty after I had graduated leaving me devastated. Even though it was my dad who inspired me to become the man I am today, I still had to move past the terrifying emotions and continue onwards with my life. The best way out is always through. He always used to say that I have had so much of unblemished potential.

We decided to go on a journey, just to get away from all of this, just Grant, Riley, a few friends and myself. They convinced me to go to a tropical Asian Pacific Archipelago called the Rock Islands. The Island was the most magnificent one that I have ever seen. The wind evoked the water to violently assault the enormous rocks. The sun reflected of the bright green leaves while my feet were heated by the sand. I was just surrounded by pure excellence everywhere.

Moments after experiencing the beauty of the island, we discovered that it was pirate-infested and we were ambushed and then taken prisoner by a sadistic pirate lord named Vaas Montenegro. We were forcibly escorted into a cave. I thought it was just a nightmare because who would have possibly imagined that we would end up in a situation like this while on vacation.

After he had left we had broken out of captivity but Grant was shot while pursuing our escape. I desperately tried to stop the bleeding but I was overwhelmed by the gunfire from Vaas and his men so I had to leave my brother's corpse behind and sprint away from the premises. I did manage to escape from them but I had fallen into a river.

I think that the loss of loved ones is the hardest blow in life. A man is determined to find justice after the loss of a loved one, even though he is incapable of fully remembering the crime.

Vaas murdered my brother. I could not let this go. I had acquired anger, grief and all sorts of emotions, mixed emotions, so I had to track him down and terminate his life. After regrouping with my friends I told them that we could not leave knowing that the man who murdered my brother still breathes. It did not take us long to find him but when we did, I did not have to think twice about piercing my dagger into his soul and watching his last expressions.

I had matured into a fearsome warrior, though my friends were disturbed by my transformation into a killer.
The island had made me a monster and there is still anger inside but I also believed that somewhere inside me, there is still something better than that. I have transformed from a fearful boy into a man who is fearless.