At least 15 seconds before she was shot a few people started shouting "He's got a gun" - and this would have been visible to her too, yet she still decided to continue to break through the door. It's difficult to comprehend any other outcome other than her being shot, especially given the personalities that were being guarded at the time.
It's difficult to tell if the officer issued a warning as there were no cameras on his side of the door, and there was a lot of shouting from the mob. Castigate him all you want (and for some reason you seem intent on blaming him) but I really think words and warnings would not have been enough to halt this mob breaking through and creating havoc. The blame lies at the feet of Trump, the baying mob, and the woman herself for being so stupid.