The person we should be moaning about isn’t the ref

[Sir C]

The elbow was 100% deliberate but not convinced it was a red card. There isn't a big swing of the elbow.

A deliberate elbow to the face is a red card p, irrespective of the length of the swing.

[Peter]

A deliberate elbow to the face is a red card p, irrespective of the length of the swing.

Not if you just 'leave it there'. Similarly, you can deliberately smack someone in the back of the head, off the ball, as long as the main contact is with the arm rather than the fist.

Violent conduct is a red card but 'he knew what he was doing there' can fall short.

In other words, refs can do whatever the hell they want. Or, more accurately, I understand that refs are encouraged to consider the degree of force in assessing violent conduct and/ or endangering an opponent and to prioritise this over intent. Therefore an unintentionally reckless challenge of excessive force is considered worse than an intentional 'challenge' of lesser force.