Why did I do it? Because it amused me, of course. When you have infinite power, what other reason is there to do anything? I take as much amusement as I can find from a world, then I destroy it and create a new one.
I've made many different worlds, and interacted in many different ways with them. Sometimes I just live and observe. Sometimes I'm a mysterious being, a witch or a fae or something, a being willing to make deals with those who seek me out. Sometimes I'm an evil figure, the world coming together to oppose me or sending heroes to fight me. Sometimes I live as a fairy, or a dragon, or some other being. Sometimes I rule as an empress, or as Queen of the Fae. Sometimes I rule the world as a goddess. And I may in fact be a goddess; if nothing else, with infinite power I might as well be. I create gods, too, sometimes, though of course they can only ever be lesser beings than I.
I don't know how long I've existed for; I've long since lost track. The universe may well exist for billions or trillions of years before I remake it, and I have done this trillions of times at least. You may think I'm bored of existence after so long, but I'm not. Life, after all, is constantly surprising. It doesn't have to be, of course; I can command every being, every object, every individual particle if I desire to. And sometimes I do; even after so long, the feeling of exercising complete control over every bit of existence is exhilarating. But it can also be boring, if done to often. So I do sometimes take complete control of everything, mold all of existence to my whims and make everything within it obey my every thought, but usually I do not. Usually I create whatever world I want to at the time, take whatever role within it I decide to give myself, and just let it play out. With free will comes unpredictability, and the unpredictability of sentient life is, well, unpredictable. Endlessly so. Especially from humans; there's no end to the surprises they bring.
Sometimes I make changes to a universe partway through just to see what happens, such as suddenly introducing magic to a world without it. Or perhaps I'll take a work of fiction from the world and make it real. Or bring about an apocalypse and see what the survivors make of the ruined world. And sometimes I'll make multiple universes at the same time and see how they interact.
And sometimes, when I recreate the universe, I'll include things from the old one, just to see what people make of them. Creatures or technology that existed in the old universe but not the current one, for instance. Sometimes I'll carry places or concepts or even certain beings across multiple universes. And that's what I've been doing with him.
It began fairly recently - by my standards, at least; it was a few thousand universes ago - in one of the worlds where I ruled as a goddess. He desecrated a temple built in my name, and I made him suffer for it, as I generally did in that world. But rather than the usual condemning his soul to endless suffering until the end of the world, I decided to take a more... involved approach with him. His life continued, but I intervened constantly, taking action to torment him endlessly. I turned his loved ones against him. I subjected him to illness. I turned his food to ash in his mouth. I took every opportunity to torment him, and then, when he finally died, I reincarnated him and continued his torture.
Sometimes I let him build a life for himself before taking it all away. I'll let him have a family, then give his children a fatal illness and force him to watch them die. Or sometimes I'll have them abandon him. A few times I've forced him to kill them.
Sometimes I make him into a criminal. Sometimes I curse him to spread pain and misery wherever he goes. Once I even made him a king, let him build his kingdom to prosperity only for ruin to fall upon it. And when I finally grew bored with that world and once more recreated the universe, I brought his soul through to the new one and continued his torture. Not because he deserved it, but because I fond it amusing.
Eventually, he realized what was happening, that I was constantly reincarnating him and subjecting him to endless torment, and tried to take action to stop it. He's tried to appease me, he's tried to hide from me, but of course he always fails. I do, though, sometimes let him think he's succeeded for a while, that he's calmed me or managed to escape my sight, only to destroy those hopes.
He's tried to kill me, too, out of desperation, but of course this is impossible. But he still tries sometimes, and eventually, after many attempts, he came to a realization. He saw a weakness, a vulnerability. He realized that my infinite power would mean nothing if he killed me before I had a chance to use it.
In this world, I was living in the forest as a fae. I was known to sometimes make deals with mortals, if it interested me, but it was only the desperate who sought me out, as while I always remained true to my word, I would often twist the deal against them without actually violating it were I left with an opening to do so. And I had just made such a deal when he struck. The woman who'd struck the bargain was leaving after doing so, and as I was watching her go, he came up from behind me and ran an enchanted sword through my head.
I fell to the ground, seemingly dead, but he thrust his sword through my head and me heart several times each, just to be sure. Then he waited for a while as my body lay on the ground, unmoving. And then, finally, he sighed in relief, seeing that I was dead.
And then, suddenly, I stood facing him, completely unharmed.
I saw his hope shatter and despair take him, and I laughed. "You didn't truly think I could be killed, did you?"
"B-b-but how? I did kill you. I struck before you even knew I was there."
"Please. My awareness isn't limited as yours is. I knew you were there. It just didn't matter. I took no action because I wanted you to think you'd found a way. But I didn't have to do anything. You didn't really think I'd take a truly mortal form, did you? I can't be killed. Even without using my power further, it's impossible to kill me."
"But-"
"But you thought I had to know you were there to protect myself? You thought you could kill me if you caught me unaware? Of course you did. I let you think that. I wanted you to think I had a weakness. I wanted you to find it and to use it. And why? Well, because it amused me. I wanted you to fight, wanted you to think there might be a way. It's more entertaining that way. And you did think you'd found a way to kill me. I left you a false opening to find, and you found it. And so, allow me to reward you."
And then I took his life from him. For good this time, though. No resurrecting, no reincarnation. I didn't even send his soul to be tortured. I simply ended his existence. I'd had my fun with him, after all, so it was time to move on. Besides, if I decide I do want to torture him more, I can always restore his existence. I can do anything I desire to.
After all, my power is infinite.
(Not mine credit goes to guyyoumetonline on reddit)