I feel ashamed of my culture because of my ex
What makes you ashamed about your culture?
My Christian ex-boyfriend spends half of the day on his computer watching porn
You should not feel ashamed because of the behavior of your ex. We cannot control the behavior of others so you are not responsible for how he acts.
I wish I could destroy him and make him feel as bad as I do.
While I don't think that your boyfriend's behavior is something for you to be ashamed of, I really do think that you should not react in this way.  It is wrong to wish harm on him like that.  If you can, try to help him change his ways, otherwise, accept that he is not your responsibility.