We know this that the game may seem difficult, but we wanted to challenge the player. The guards are very well trained - after all, they conquered almost all of Gaul together with Julius (well, except one village xD).
Player has no chance in a duel against a guard. The only way to eliminate them is to attack from behind :)
Sneaking allows you to slip behind your opponent's backs. They can still see you, but they can't hear you - your steps are completely silent ;)