Not reporting it in to the cops leaves the idiot at large. Assuming you don't know who it is, that leaves him to possibly kill someone else when he does get his aim right...
If that happens, the question is: can you live with yourself knowing someone died, because you didn't call the cops due to the fact your phone was broken, and thought it was too much trouble otherwise?
I'm not saying that reporting it in will get him off the streets on time before he may kill someone, but atleast you could say to yourself that you had done about all you could in the given situation.