Killing people is just another facade of her's. If Rin herself had to face down someone, to take their life with her own hands, then she may falter. What she did before was not of her free will, she suspects that much, but since she was a monster...

"They would just do the same to you. Please Saber, I rather not trouble others with my battles."

At least she can still stand tall. Matou or Tohsaka, Rin took that part of her very seriously.