The way I'm interpreting this is that Rock's immense hatred and animosity towards Cody stems from him channeling the genuine love and admiration of the fans away from Rock.
The fans that were his first, and also feel have switched up on him since his return causing him to turn heel in the first place. He thought being the Final Boss would gain him back the fans' favor, but they still largely see him as the old guy or "Dwayne" showing up to insert himself in spots he doesn't belong. It's pretty meta.
Now that hatred has turned to obsession; he wants to either control what Cody has earned and feels he has lost or stump it out by any means necessary.
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