When orders come from a figure of authority we can easily deny personal responsibility because it is assumed that they will take ultimate responsibility
Most human societies are ordered in a hierarchical way, where some members of the group have legitimate social power above those beneath them in the hierarchy
We learn via socialisation that we will be accepted if we obey those who have authority over us (we trust them and/or because they have the power to punish us)
It is legitimate as it has been agreed by society that these positions carry power and most people accept this
In countries where obedience and deference to authority are less valued (such as Australia), obedience rates are much lower than in countries that value legitimate authority figures (such as Germany)