If you want to do anything about the stealing and killing in guarded town, you should involve reputation.
Killing for instance, -20 for for normal npc, -25 for traders and flc staff, -30 for guards, rangers and doctors and -40 for leaders like Tandi and sheriffs.
The same with stealing, every attempt noticed by the guards would result in -5 or -10.
For NCR doing such things inside the city would mean a bigger negative reputation than doing it outside.
Off course your reputation would increase with Hub criminals, raiders, etc.
Any rep lower than -300 should cost you to lose protection in guarded cities.
Still it wouldn't stop people from creating alts or increase their rep. by killing.