Honestly, I saw nothing particularly wrong with your story except for a couple word choices and one very specific action. Other than that I think it could work, and maybe even in YA.
I think the problem here is that you seem to be very, very focused on the sex, violence, and drugs. I'm not going to say that those things aren't slightly interesting. They can be, obviously. You mentioned things like Grand Theft Auto. Sure, it's hugely popular. As are movies that are similar. But, at the same time, when someone sits down to read a book, they're not looking for a novelized GTA San Antonio....
On this thread we were trying to discuss edgier YA fiction that's truer to life and maybe experiences that people our age might go through. Yes, there probably are rapist cops out there, but I'm not sure how many people want to read about said cops trying to rape a girl who slits their throats...Maybe it all works out in the story. Maybe there's some characterization later. If so, then you should definitely ignore any negative comments, if you feel it's really needed. If not...then it seems like you're just putting it there because you think that's what teens want now.
As others have pointed out, you're generalizing the teenage population. You mentioned that you're 25 (I think) but you seem seperated from the diversity that prevails at all ages, including teens. Not everyone likes seeing people shot for no reason...
So, just to wrap things up, I don't think people found your writing terrible. I think they just didn't see the broader picture. If there is one, then great. If there isn't, take a long look at the piece and see if you need it. As to the teen comments, be careful generalizing. It's not appreciated, as you found out, even when you might be joking. Jokes don't work well online anyway. Especially when the joke is potentially insulting.
At this point I think we should all just...I don't know, forget about things said and start over and get back to the discussion about edgy YA literature.