Case #3 [A Bloody Dance]

30 minutes before the competition, A contestant named Mary were found dead in the costume room. lights off, too messy and bloody. the way of killing here was too harsh. the police suspected 4 people.

[During the Interrogation]
Tania(her friend)
-I was the one who saw her, I can't do it to her, besides, she is my friend and she is the best dancer for me. I am so9 disappointed. She is like a sister for me, I will miss her.

Raphael(her boyfriend)
-when her friend screams, I immediately ran into the costume room. when I turned on the light, I saw hee dead body. I can't do this to my girlfriend. I promise that the one who did this will suffer.

Shane(her opponent)
-I am preparing because I am the next contestant, when I heard Tania screams, I ran into the costume room with Raph, He turned on the lights and we saw Tania crying over Mary's dead body. Well, I can't do this, yeah she's my opponent, but she is also a friend of mine.

Morgan(her make up artist)
-I went to the judges to give the paper with Mary's name. when I am on my way to the costume room, I saw Raph and Shane running. I asked them why and they said "Follow us, something happened in the costume room. and we saw Mary's dead body with her blood spreading on the floor. I can't do this to her, we are really close to each other. I am so sad to see her bloody death.

After the Interrogation with the four suspects, the police immediately arrested the murderer.

Who is it?
Why?