The security, antitheft or lock picture/icon is a visual indication that something in the car's antitheft system is failing. In most GM cars and trucks there is a hidden system that most people are unaware even exists. From the moment you put your key in the ignition, there begins a constant line of communication between your key's security chip or transponder, to the ignition's lock cylinder, and then to the car's TDM (Theft Deterrent Module). These 3 parts are in CONSTANT communication. If the key is in the ignition, then these parts are sending signals to each other. The security light illuminates when that signal gets broken, even just for a second. When the signal breaks, the car misinterprets this as an attempt at theft and goes into antitheft mode. The car thinks its being stolen or hotwired because it is not getting that signal from YOUR specific key. This is why the security light turns on and shuts down your vehicle. This link explains it pretty well.

http://www.diy-gm-security.com/

http://newrockies.com/

The ignition system in most factory installed security systems is disabled when the alarm goes off so that a potential thief can't start the car and drive it away after the security system has been triggered.

I would think that is a feature of the security system. You know, so it can't be stolen.

It seems to me that a "security" system is keeping the vehicle "secure". If the alarm is doing it's job, it warns, and keeps the vehicle from starting.