I truly don't think you're a bad, evil power hungry DM. I honestly don't know how you actually DM, but I'm fairly confident in saying that you're probably not that. You consider yourself a flexible and fair DM, and in practice, I can believe it.
Within the scope of this conversation, surrounding a simple hypothetical, what you've shown is a lot of inflexibility and a need for the player to concede without compromise. That's all. I don't think this makes you a bad DM at all.
This isn't the first time you've switched things around to make things more of an affront to you personally, so I do honestly wonder if that is a cause for the inflexibility presented.