Sayaka's characterization makes sense. She
tried to do things the idealistic way, and it only left her as a horrific bastardization of nature while basically robbing her of everything she loved. She said "Screw it" to life, and had an established reason to do so. Maybe I'm being too partial because her characterization is reeeeeeeeally similar to how I wrote a certain character in a certain route in Terrible Things Happen to Akane, but Sayaka makes sense to me.
Kyoko's characterization also makes sense. Through witnessing Sayaka's downward spiral, she saw where her own life was heading and decided to change sides. She befriended Sayaka out of sympathy. Abruptly being forced to face your own mortality generally has a big effect on you.
Sayaka and Kyoko are only nonsensical when you ignore the valid reasons they act the way they do.
Homura is the only one who currently makes no sense, but I hate every aspect of her except for that
one thing she did and will continue to think of her as the show's
Mashiro until she
explains things. Not much to say there.