Okay, there's quite a few things wrong with this one.
1) When you introduce a character, you don't put their name in () after the sentence you introduce them in. Put them in the same sentence. Such as...
KARL, a thug in a black baggy starter shirt and sagging pants, stands in a huddle with the rest of his gang.
2) The dialogue needs a lot of work, especially when Karl and Mike are talking. You are trying really hard to make them sound like gansters by adding slang terms to their dialogue. Its a good thought but it ends up sounding forced and juvenile.
3) Why does Jaron want to go find the serial killer, while Karl and Mike are scared out of their wits? And when Jaron is in the house, why is he talking to himself? These are things that you have to watch out for to make the story believable.
4) Why do Mike and Karl have a police scanner at their house? This is a little far-fetched.
5) When you change scenery you also have to change your slug lines. Towards the end you have "INT. POLICE STATION" then you go on to say that there is a chase all the way to the hospital. If there is a change of scenery you need to express it with a new header. Also, you need to explain the chase. Was there a huge car chase to the hospital? Did the shadow get caught only to escape again? You can't just say all of this in one simple sentence "they follow it to the hospital..."
Even though there were some problems, this wasn't that bad of a script. Some of it was actually written pretty well. Keep up the writing, while being mindful of writing realistic dialogue, and you will keep getting better and better.