canvasback,
I don't think the shooter padlocked the exits, himself. The fact that they were padlocked, is likely one of the factors he used to choose the club, together with being a "gun free zone", without adequate security. Many venues such as this padlock the exits to prevent people sneaking in. It is true that the code required "Panic Hardware" prevents opening the doors from outside, but law requires they not be locked from inside. This means one person could pay the cover charge or ticket fee and let all his friends in. To prevent this management often just padlocks the doors, rather than hiring enough security to stop "gate crashers".
Mike