Well, this is known as a PK I system, or PassKey One system......
What happens is, when a key is inserted in the lock cylinder, the inside contacts of the lock cylinder touches the resister contacts of the key, completing a circuit back to the PassKey module.....if the module "sees" the correct resistance, it does two things, it grounds the starter enable relay, and it sends a fuel enable signal to the ECM, to fire the injectors......without this signal, the injectors will not fire and the car won't start if you jump the starter.....
What GM has, is a tool called the interrogator.......what this tool does, is read the key while it is in the ignition, by disconnecting the harness that goes up the steering column to the lock cylinder, and attaching the harnesses to the interrogator.......now it reads the resistance and tells you, which key is in the ignition, by showing a number between 1 and 15.....for example, the interrogator may show 13......that means you have a #13 key.....now if instead of a number, it reads "E", for error, that means you do not have a complete circuit up the steering column, and the car won't start....this is what usually happens when you have this problem.......that wire harness that goes up the column attaches to the lock cylinder, and every time you turn the cylinder, those wires flex and can easily break....when that happens there is no complete circuit and the PassKey module can't read the key.....
So here is what you have to do......one of those 15 keys should work.....take one key at a time, insert it and turn key to "on", not crank.....the SECURITY light should come on, and then go out after 3-5 seconds.....if it stays on, that is the wrong key........remove the key, and try another......
THIS IS IMPORTANT! Not sure if the Locksmith told you this, YOU HAVE TO WAIT 3 MINUTES FOR THE LOCKOUT TIMER TO RESET.....If you try a key before 3 minutes, even if it is the correct key, the vehicles will not start......you must wait a minimum of 3 minutes between trying each key....
If you try a wrong key(has incorrect resistance), the light stays on....... when you turn the key to off and remove it, you must wait 3 minutes before you try another....when you insert a key, and turn to on, and the light goes out after 3-5 seconds, that is the correct key, with the correct resistance.....
The locksmith cut you 15 keys? I think one key costs $35(cut) at the dealership.....how much did he charge you for 15 keys?
You say you had an aftermarket security system on the vehicle? Did the Locksmith know this? Because this system had to have either a key or the correct resistance attached to it.........
Now, let's say, you tried all 15 keys, and the SECURITY LIGHT did not go out....I doubt the wire(s) is broken since a new lock cylinder was installed.....however, since the locksmith was working on the car, it's possible he may have forgotten to reconnect it......you will have to drop the hush panel on the driver's side, and
look at the base of the steering column, where all the wires come out.......look for an orange sheathed harness with two white wires coming out of it going to a black connector......that black connector should be attached to another black connector with two white wires coming out of it......it looks like the wires in this picture:
Make sure those black connectors are attached to each other....
And like I said, you must wait 3 minutes(I would wait 4) before trying another key.....
Also, one more possibility......you did remove the aftermarket security system.......are you sure you reattached all the wires correctly and soldered them?