Best of both worlds? As alarm progresses there is a roll to see if the executive gets a gun?

Another option could be to make them become armed after a certain amount of time has passed in your current playthrough, such as after 2 days or something.

Give us 3 options.

Option 1: Executives are always armed.
Option 2: There is a 50% chance for the executive to be armed.
Option 3: Executive becomes armed if Alarm Level exceeds a certain amount... say, Alarm Level 3 or 4.

In regards to the third option:
At low alarm levels the executive is not very worried yet, he hopes it's a false alarm or that it'll be handled by the guards. But once the alarm level rises too high he realizes that it is indeed not a false alarm and that there is something terrible going on. His response? He goes and gets his gun from where he keeps it in his desk drawer and prepares to defend himself. I do not know if the third option is even possible, but it'd be awesome.