"Yeah, I might be killing you but I'm actually saving you!!!!" At least Homura's logic for trying to kill Sayaka before she turned into a witch was sound. She had the experience necessary to realize that there was no saving Sayaka now and taking her down now would ensure that neither Madoka nor Kyouko got hurt in the process. Homura, unlike Mami's incredibly emotional and irrational response, turns to killing as a final resort when nothing else will work. And she doesn't let her emotions judge when that is.
I wrote a 500 word prompt on Mami's story, explaining why she would break down and do emotional and irrational things when I realized that Mami is the coldest and most rational character here.
Instead of killing everyone, which would definitely 100% save them from the fate of becoming a witch and guarantee that they will never ever kill anyone else as a witch, she should have thought about how killing them would literally be killing friends, and chosen to not kill them to live the rest of her life in fear of seeing her friends or herself turn into a witch, which might just end up with more dying anyways fighting that witch, on the hope that they can find a solution and live happily afterwards.
That seems to me as way more illogical and emotional than what she does, which is basically the most ruthless method to guarantee that no more people will be harmed. If I kill myself and my friends right here, nobody else will die and we will never suffer as a witch. Even in her insane state she executed her plan with precise, rational actions. Kyoko was the largest threat, as Mami didn't know if she got her illusion magic back yet, so she was the first to go. Next Homura is wrapped up, nullifying her time magic. The only thing she miscalculated was Madoka's willingness to kill her.
Mami has more complexity than you give her credit for and hating her for this one moment because of her "emotional and irrational" actions when Homura was basically doing the same thing with Sayaka is pretty hypocritical. I started out not liking Mami as she was always the one who gets in Homura's way but after reading her story she's a character I can understand and feel for and I cannot dislike her more than anyone else in the quintet.
Plus again I highly recommend reading The Different Story to anybody interested in learning more about Mami + Kyoko.