A murder investigation, but there's a big secret

You and your partner Nick are investigating murders, there were several of them, but they were very complicated.
One evening, a girl named Casey was killed and you began to investigate the case. She was stabbed three times and they began to strangle her, it was evident from the marks.
Nick has been your partner for 4 years, you tell him everything except one secret that no one knows... Except the victims, and not even all of them, because before you kill people, you put on a mask, and no one knows about your secret.