1 I'd guess the last one.  Misaka's face seems to be tinged with sadness, as though she was forced to do something that she really didn't want to do.

2 Haven't watched all of Asobi ni Iku yo, but Eris hasn't struck me as likely to kill anything but maybe a mouse, so I'm guessing that one's off the list.

3

As far as I know the Railgun never killed anyone. And yeah, Eris never kills anyone. (She beats up and breaks a bunch of doggiebots in that scene, however.)

It also isn't Diane. (The bad guy doesn't die, though you only learn that if you follow the manga.)

The answer is #3, Anya and Tsugumi. Shortly after that scene, the two of them along with Meme kill Shaula Gorgon, in Soul Eater NOT with the "Spring Bird Attack."