I had a fuel pump replaced and after the job was done my battery was dead so It was taken out and charged up now my anti-theft light and the car won't start,days' later I came out turned the key it started,ha! i thought,,while the car was running I opened the door and the car cut off!

Is the security light still on? a quick test is to remove the negative terminal on the battery for 10 minutes. reconnect and see if car starts without the security light on.
if the light remains on turn the key to ON without starting engine and leave it for 10-15 min until light goes out and then start the car, it should then stay running when you open the door.
the late 90's into 2000's GMs have issues with their security systems acting up, unfortunately your car is one of the ones it affects.there is no way to disable the system because it is part of the ECM
always unlock driver door first when using the key, there is actually a sensor that the computer reads, if the door is unlocked without a key such as using a slim jim or coat hanger the system will trigger a no start and the light will be on.