The reason Peko murdered Mahiru was because she never felt like she repaid Fuyuhiko for him saving her life time and time again. She never knew how Fuyuhiko felt about her and thought I'd she did what she did Fuyuhiko would return the feelings without making her seem weak to the others. She knew that she would never be able to see Fuyuhiko return the feelings, but when she saw Fuyuhiko at her execution she knew she could get her last goodbye she knew she had to save him one more time and died with a smile on her face.
