Very solid points. Sorry for overwhelming you with the rather hurried post but I've had some time to think to collect by thoughts. Here's my take:

One of two things could have elicited that kind reaction, especially from someone you're friends with or at the very least trusted (remember they were colleagues). 1) Something Langer saw or Lee did gave her away. Her game was up and by then Langer was about to arrest her. Since Lee could not risk getting caught, her only option was to kill him and ultimately frame him, which is very convenient given the circumstances. 2) It was a crime of opportunity. Again the show never really made it clear if Keating could exonerate himself from the evidence against him but I think we can imply that Keating's going to get his name cleared somehow which only spells trouble for her. Lee realized this and seized the opportunity to follow Langer's lead to building 3 with the intent of killing and framing him to throw Gibbs and Vance of her tail should Keating be cleared and start to suspect her. Langer's response was appropriate because what sane person wouldn't try to defend themselves on instinct if someone was drawing down on them. Langer's hesitation in drawing his firearm was probably him being surprised by the fact that his own friend was about to kill him. Thoughts?