I would think that selection, training and control of these guards are the cruical point.
There were long bitter "gunfights" between terrorists and armed guards in the past: Remember the several bloodbaths on Aeroflot aircrafts back in the 80´s soviet times, when the armed cockpit crew or guards tried to overpower hijacker at gunpoint, which lead to long gunfights between them and terrorists (and the fare paying passengers in the middel). Or that infamous battle between palestinian terrorists and iraqi secret service guys on board of a Iraqi AW B737 in 1987, which resulted in many deaths and the crashlanding of the aircraft in Saudi Arabia. What I know from these occurences let me ask, what thugs are be used from the UdSSR or Iraq for this important and sensitive job.
On the other hand, the "Skymarshal" procedure seems to work very good at Israeli civil aircraft. But Israel (as far as I know, correct me if I´m wrong) have very due diligence and harsh training for their Skymarshals and cabin crew.
In short: As SLF I have no problem with armed guards, as far as I can trust them.