and I doubt she wanted Mami dead, but that didn't stop her from killing her. Something can happen along the way that Homura needs to be stop one way or another, death can be an option. Maybe thats why Homura said that they will fight. Maybe she wont be able to be sane and will act insane or something. I doubt Madoka will say. "Hey Homura, can I have my powers back. We can still be cool with each other." "Yeah no problem babe." Homura wont allowed to thing go back the way it was, even if it means to get in Madoka's way. Though I doubt Homura will kill Madoka if she stays sane through all the next movie.
But Homura did understood Madoka's wish and was not ok with it, while the others three were ok with it and they most probably understood her wish. Wasnt Homura being selfish then from wanting the best for Madoka and not respecting what Madoka wanted? Even if you say that Sayaka has the best life ever in the new world, if it's something she does not want, then Homura has no right to say what's best for her. Just like Madoka had no right to do so, of course she gave Sayaka the choice to stay alive or not.
Its not about not wanting, its about not knowing where to start. Homura pretty much gave Kyubey a huge clue where to start. I should've worded that better, my bad.