Cunningham: For starters, the power of Shazam is big-time power to wield. Also a tremendous responsibility to use. And it’s all in the hands of a 15-year-old kid, which — I don’t know about you — but if this were the real world and a kid had access to that power, it’s enough to give anyone pause. So John is naturally very wary about Billy Batson wielding this enormous power irresponsibly.
So, “Constantine being Constantine,” he steals the power for himself instead. But what he finds out about the power surprises even him. Oh, and the power of Shazam hurts like a mutha if you’re not the chosen one to wield it. A small price to pay, though.
And there’s another reason John took Shazam away from the main “Trinity War” scene. One that ultimately backfires in so many ways.