If you are walking down the street and an aggressor approaches, leaning into your face, what would instinct dictate you do? You would, on impulse, defend your personal space.
Constantine Hatzidakis was not walking down a street, of course. But reflex does not make allowance for location. In feeling the presence of Andy Robertson, physically and verbally, the assistant referee has attempted to brush him off.
Yes, the angle of his elbow does not look good. But there is a big difference between a pre-meditated elbow intended to cause harm - a motion with your arm in a horizontal position - and the act of self-defence which sees Hatzidakis raise his arm towards the Liverpool defender.