Kyle wasn't obligated to carry an assault rifle in a crowded city. He travelled there with the hopes of discharging his fire arm. A dream of his. To finally have an excuse to fire at real people instead of the boring target range.
In fact Kyle tried to convince shop owners they needed armed security and the shop owners refused his "help" in advance. He was an armed instigator there to provoke violence so he could meet it with justifiable deadly force. Keep in mind he's not a cop, or security guard, or military. Just some stupid kid with an itchy finger.