Q: Why doesn't he kill the girl?
A: Because he is mesmerized by her b
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bz. Aren't we all?
I beseech ye in the bowels of Christ, think that ye may be mistaken.

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My password is password.

I don't remember that he said "subconsciously". He says it wasn't suggested in the script, but he decided to interpret the character's behavior that way, because it seemed like the only logical explanation. He says that the character is in love and hence "doesn't know what he's doing", which doesn't mean that it's subconscious, but that he isn't behaving "rationally" any longer. It may certainly be a novel feeling for him and hence he doesn't quite know how to deal with it, certainly also fights it because it gets in the way (even to the point of endangering his life), but that isn't the same as being unaware of it. In fact, once you realize this yourself (personally I suspected it upon first watching the film, but wasn't sure whether it was my wishful thinking :-)) and watch the film again, there are many hints at what is going with him, and much sooner than the third act.

According to John Hurt in the Criterion commentary track, his character "falls" for the girl on a subconscience level that even he doesn't realize. But he does begin to admire her in the third act.
D.

He intended on killing her, but the fight she put up at the end gained his admiration. "You're a very luck girl," he says after he he knocked her unconscious.

That's a good question. He certainly wants to, but I think the problem he has is that she's so innocent and pure. Unlike Willie, she didn't betray her friends and unlike Myron she doesn't hurt people for the thrill of it. Myron keeps asking for Mr. Braddock to save her, but in the end he gives up because "it's a job".
What's interesting is that if he had killed her, he probably would have gotten away with everything and made it through as a backpacking Parisian.
They're funny people, the Italians. Culture really isn't their thing.