[Max Dexter]

Originally Posted by Nayrael

Ein is a mistress of deceit, Zwei is a Master of Strategy and Drei is a Mistress of guns... or she would be if she didn't have that tendency to play with her targets. When she is serious, she can be overpowering (as seen in Cal's route in the VN).

As for why she didn't shoot...

Spoiler for VN spoiler:

...she intentionally let Reiji kill her by going easy. If you had sex with Cal (before Reiji's reunion with Elen that is) and went for Ein's ending, Reiji doesn't have the heart to pull the trigger so she kills him instinctively and is surprised because getting killed by him was part of her plan.
Why she was like that was never explicitly stated in Anime, though you may make a guess.

Reiji decided to save himself and Elen and sacrificing Cal is required for them to reduce the risks.

Spoiler for VN spoilers:

Though you are correct, as shown by Mio's route where Reiji's decision to save them both ends up in both Ein and Drei surviving.

I doubt it, because when Cal asked him in the church whether he is there to protect Ellen he said it's his "duty" to kill her, i am assuming that what he meant is to correct his mistake and take responsibility. As unfortunate as it seems i think that Ein did matter in the decision at the end because he said to her before in episode 22 that she can kill him but she has to let Ellen go. Though i can't understand why killing her would be the best thing for her according to Reiji. It's said but i think all he did was feel sorry for Cal, and decided to pursue Ellen's dream.

I don't play games neither i read LN, i only watch the anime version which i didn't like.