There's usually someone you'd rather not shoot with at ranges that are open to the public. There may have been someone else at the same range with the OP. From his perspective, he was upset because his concentration for some serious practice was broken by the OP freaking out because of the alleged gang banger.
I always suggest shooters familiarize themselves with a range's rules before they sign-up to shoot. Look at the rules two ways: are they too restrictive for the type of shooting environment that will make your comfortable, and are they restrictive enough for the same reason.
To avoid the machine gun wannabes, some ranges require a fixed amount of time between shots, three-seconds for example. This may be fine for you, it will keep out the riffraff; but, the rule doesn't work for those practicing a course of Rapid Fire that requires a 5-shot string in 10-seconds.