A secret service agent is undoubtably trained and competent to use firearms, so are SAS etc etc.
However all that is required is some false paperwork and an armed bad guy is on board. Photo ID's can be faked - unless the agent is personally known to the captain it is impossible to fully prove his bone fides.
There can be no reason for a passenger to have firearms in the cabin. The safe and only option is for all firearms to be hold loaded.