How do I delete this?

I'll just lock it for you. In future, just let a mod know. As Emma said, it's unfair to delete a post after a review has been made.

-Blink

In answer to your question = it won't. Ever. If someone's already reviewed it's unfair to suddenly take it away from them. Though it sucks. I know.

~EmmaJane~

This is very short, so not much can be said. Before killing a character off, it's best to make the reader love or hate that character so that they'll care. Otherwise it's not really a person dying, it's just a bunch of words dying. The only remedy is to make it longer, so you have time to build a bond between the characters and the reader.

The knives strapped to her leg, you didn't mention were hidden. Did she have a skirt on to hide them, or were they inside her trousers? Details.

There isn't any description of the setting, or the characters. I don't know what time of day it is. The right sort of description would help set the atmosphere.

With her exit, you mentioned she left a trail of blood. This would be a dead giveaway of her identity when the police show up, and if you want to make her seem like a professional killer, she should be well aware of things like this. Her fingerprints would also be all over the boy, which would also mean the police could nail her instantly. A professional killer would take into account all of these details and devise a plan that leaves no evidence. At the moment she seems like she doesn't know anything about being an assasin.