Help with plot of my story?

I have an except, but no plot. The idea came to me on the bus actually, and I wrote it all out in my phone as quick as I could. I like the except, I just want to develop it. I'm wondering if someone can help me a little bit with where to go from here.

This is the except:

Blood. Warm. Crimson. Red. Everywhere. It was all I could see. Everywhere I looked - red. Looking down at the delicate blonde girl with the pretty blue eyes that were frozen in shock, I could only see red. The red liquid pooled around her, making intricate patterns as it travelled down the damp and sordid alleyway. The smell of her strong lavender perfume mixed in with the coppery smell of blood assaults his nose, along with the putrid stink of urine and garbage. He reaches over with his gloved hand to stoke her beautiful golden blonde hair. Bending down he sinks his crooked nose into it, taking in the scent of flowers before quickly plucking out a lock of hair and pocketing it. The girl had been stronger than she looked, her long legs fast to react as she tried to get away from him. Silly girl, didn't she know that he got everything he desired; once he picked "the one", they could never get away from him. He had to admit, the chase had been quite thrilling though. She most assuredly made him work to woo her; quite a change from the others that threw themselves at him. What was the fun in getting everything handed to you on a silver platter? He liked to work for it. He loved the chase, and the excitement that he got from it. The reward in the end was even better at the end. As he had slowly slid the knife across her silky throat, he had enjoyed seeing the determination and fear in her eyes. She tried to fight him, she really did. Damn girl had left quite a lot of scratches on his back with her long nails, but her attempts were futile in front of his strength. She had fought with him to the end, as he watched her cheeks go pale and the life force leaving her blood. It was a beautiful sight to him. Reaching into his breast pocket, he pulls out a battered up digital camera. The bright moonlight shined down upon the girl, much to his advantage. Positioning himself in just the right spot, he starts snapping pictures of the scene in front of him, taking snapshots of the patterns of the blood as well. His face drops as he realizes he has to finally depart from the girl before someone sees him there. She really had been a fun girl to entrap in his plan. Pulling out yet another black rose from his pocket, he slowly lays it on her unresponsive chest and walks away, never once looking back at the girl in the back alley.