Killing was something she did during the war and something she had to do occasionally as an Auror. She never had a problem with the act itself. Hermione knew it was easy to do, and the fact that she was so nonchalant about the fragility of a life she took, was what had bothered her. Even now, she wasn't shaking because she could have killed Ron, it was the fact that her boss and best friend- Harry Potter himself- was going to be livid when he found out she lost control and nearly killed Ron.