>>132851170>>132852041Because AI YO.
To expand upon that, QB explained it towards the end of the show. Basically, Homura's only way to survive and not fall into despair was to obsess over Madoka and delude herself into thinking that she could save Madoka. This, coupled with a good 60 or so timeloops, made it so that Homura's only point of existing was to attempt to save Madoka.
Now, let's assume you're still a lovely little kid at the age of 12 or whatever they are. Now imagine seeing your best buddies get mulched, splattered, gored, and turned into eldritch monsters. Now repeat that about 60 times. Now compound all of that with the grim realization that said mulching, splattering, and goring will all occur on such a basis that you can time it flawlessly.All of this combined made Homura batshit insane to a degree that is absurd.
Now once you throw Rebellion into the mix, you have to understand that Madoka told her that going away to be God or whatever else would be beyond comprehension in terms of pain for her. Homura likely tied this back to how Madoka said it was beyond cruel that no one would remember Mami (back in the original show/first movie). Homura, being the general nutcase she is from this whole mess, thinks that God Madoka is probably in horrible pain as a result of this. In reality, she actually might be, but she might not be too.
Either way, Homura rips her out of heaven and locks her in a false world (yet again), only this time, Homura has gone psycho mode (as evidenced by the change in her face/behavior). And of course this all ends in the cliiff jump scene at the very end, along with the half moon which probably shows she's fucking broken without the "real" Madoka around (the Madoka in the false world is just a stripped form of God Madoka, as shown by Homura making sure she doesn't remember who she "really is").