Factual error: Clooney refers to the rifle as an "M14". The Rüger Mini-14 is a different rifle than the M14 (used e.g. by the Marine Corps prior to the M16).
Factual error: When Clooney and his 'contact' test out the Mini 14, they fire at brushes, showing that the scope is not centered. Instead of making the windage adjustments, they actually adjust the top of the scope, making elevation adjustments.
Answer: Because she didn't, but once she saw evidence of who/what he was, he had to kill her to protect his identity.