Shazam is a comic created in the 1940s by C.C. Beck and Bill Parker. It is one of the most successful superhero comics right from then.
The reason for Billy shocking of betrayal :
From a long time wizard was not featured in the story. Then all of a sudden he shows up taking on Billy Batson’s father. Wizard chooses Billy Batson as the hero and gives him the powers. Thinking of wizard, Billy Batson sees wizard attacking his father. Then Billy Batson shows up to the scene. Wizard explains to him that his father is manipulating him and weakening the powers of him. He later blames himself saying that he was to choose Billy Batson over Black Adam. After hearing all this Billy Batson takes over the Wizard.
As per pictured reports have come that wizard is not like the one shown here. He is not that villainous. As in the world of magic, anything can happen and most of the things are deceptive this could be also one among that.
Past reference :
Wizard was referring to Black Adam as in the past wizard has given the powers to Black Adam just like the way he has awarded them to Billy Batson. But Black Adams used them against wizards’ thoughts and became the central antagonist to the story.