In this case, all Walt had to do was turn her on her side, or help clear her airway. No danger to him whatsoever. He chose not to because either
1) He hated her and what she was doing (to be more specific, blackmail)
2) He wanted Jesse to return to work and thought she was a bad influence. (Walt would make more money)
Walt chose to leave her die when a simple movement of his arms and a "wake up wake up" and clearing of her airway would have saved her and most likely, she'd be alive.
I always hated Walt for it and to me THAT is when he really became Heisenberg.. he let someone die by inaction. Sure he would soon murder people with firearms and his car but that's when it started. He let a young woman, who could have done anything she wanted with her life, had she made the effort, die. And it ultimately led to the plane collision. Walt may not have been a medical doctor but common sense for an intelligent chemist (brilliant, according to the plot) would be to save her; he chose his own interests over it. I really liked her character too and felt that she "completed" Jesse's character.
If you recall there was a scene later in the series when Jesse is crying on Walt's shoulder saying "I loved her..... LOVED HER...." and just sobbing.. on the very person who let her pass into the darkness, forever.
I suppose in a sense Vince Gilligan and company succeeded in their "fall from grace" underlying main theme with Cranston's character. I rooted for Walt since day 1 and what he did, well, what the writers did, was turn me against him. He was the protagonists, and has become the antagonist. I'm not exactly rooting for Hank either, as I think the DEA is a bunch of bullshit for the most part, especially in their relentless pursuit and incarcerations of those with marijuana convictions, Skyler I am neutral, the only innocents left are Walt's children. Even Marie had that weird ass episode where she makes up stuff about herself and stole items from open houses. The children are the only ones left. And Jesse. He's on the fence. Regardless, great show, great season, and I can't wait to see what happens.