What having a sky marshal really means

I hope your suggestion wasn't serious- remember when terrorists took over a
theater in Russia and the special forces sprayed in a "non-lethal" disabling
agent? Around half of the hostages also died from the effects as well.

What makes you think a significant number of the elderly or less healthy
passengers would also die from hypoxia? How many would suffer permanent
brain damage, or strokes or heart attacks?

I think I would rather take my chances having a trained air marshal on board
even if they had to start shooting Glaser safety slugs around the cabin. If
a terrorist does not control the flight deck they do not control the
airplane, and pilots are trained to keep the door closed and locked under
any and all circumstances.