The next Phantom Thief Makoto needed to get rid of was Ann Takamaki. However, she was almost never alone. If she wasn’t in a group, she would be with her friend Shiho, making her “cleansing” hard to enact.
Finally, she had gotten impatient, deciding that Shiho would just have to be collateral damage, taking her out in front Ann just to show her exactly what awaited her for ever daring to to rebel.