Must add my 2 sestercii worth here. Juvenal's point in the Satires was not that the guards could be bribed, but that the untrustworthy wife was as likely to go to bed with them as with anyone else.
The very people you hire to protect you may be the source of your greatest danger. Think of all the military dictators who threw out European colonial powers, only to loot their countries and oppress their people even more than the former governors did.