Wait, i change my murderer guess to Inverted Rainbow, because the pizza was made by her to keep everyone from going somewhere. After all, she is a cook.
So yeah, i Inverted Rainbow talket to Skyangel, so i think that's when she stole her mask.
It obviously is Inverted rainbow
Inverted rainbow first hided herself behind Red's bookshelf, after stealing his camera to make him go to his room. When Red arived to his room, Inverted rainbow pushed the bookshelf, which hitted Red, and killing him. Inverted rainbow left Red's camera and the gloves she used in Red's room, then ran away.
Also, i think it's Inverted rainbow because:
1: Skyangel's doctor mask is missing, and the person in the Video tape was wearing a docter mask. I explained last time how Inverted rainbow could have stolen Skyangel's mask.
2: All stuff from last investigation.