I am wondering if we are going to get any more clues regarding the shooter. Right now, I think either Ian did it, or he hired someone to do it. A second thought is he either thinks Kate did it and wants to protect her, or he knows who did it and is trying to protect that person.
Lexie dying from complications of the toxic gases seems to have been foreshadowing of what was to come. Explosions from the gas will now kill more people.