I think the actual killer was one of the people using the urinal, he/she saw Phil come in, snuck into the stall, and then stabbed him through the cubicle wall. I don't think he/she was expecting to kill him in the bathroom. It's just he/she had to go to the bathroom (if Mrs. Loomis she'd want to seem like a guy killer and used the urinal) and thought "ah hah! Now is my chance to kill him!"
"Gutted, slashed, the guy ain't in the union no more!"
Joel to gale Weathers in Scream 2 (1997); Dimension Films