Okay. I'm going fully trust your version of the story and assume that the trainer indeed is maliciously making people feel shit about themselves in order to sell his services.
How does one respond? Like an adult. Tell him straight that his bullshit sales tactic is: (a) ineffective; (b) hurt your feelings / made you angry; and (c) damaging to gym-goers there because you're all there to improve yourselves and his shit-talk will just discourage them instead.
Acknowledge that although his "honesty" might have some truth to it; it's still not the way to talk to people, much less potential clients.
If he is indeed an asshole and ignore you or threaten you; you tell the gym owners of his behavior and ask them to talk to him. (No; don't invent shit up to get him canned/banned. You tell the truth.)
If he is the owner of said gym; you pack your shit and find another gym. With his shitty personality, he'll probably soon be out of business anyways. If it doesn't and if he doesn't change? Meh. The world doesn't revolve around you.
Him being jacked isn't relevant. Maybe you were intimidated, that's okay. But you're only feeling powerless to him because you want to force him to your point-of-view. Again, the world doesn't revolve around you. You discuss and try to resolve issues like an adult.
Him being jacked might actually be relevant in being a trainer. Maybe he does have something you could have learned from if you just saw past his poor social and sales skills.
The damage you did is plenty. A couple that quickly springs to my mind:
1.) Cost + Time. He has to get a new domain and hire someone again to change up most his content to match the new domain and 'security'.
2.) Your fuckery could have denied people who wanted / needed his personal training services. Not just lost sales.
Not to mention the anguish you put the trainer through. For, at worst, your bruised ego? For something that could have just been as simple as a miscommunication.
Again, OP. That was disgusting what you did. I hope to whatever gods there is that you don't ever do that again.
If you're half the man of power and righteousness you claim yourself to be, you'd apologize to the guy and try to make things right.