It's time to get rid of the far too common perception that “Write what you know” has to be taken literally. Granted, if you write a non-fiction book on lace-making or beer-brewing it comes in handy to actually know what you're writing about, but as far as fiction is concerned you shouldn't take this line literally, but instead as a cue to dip into another kind of knowledge.
So back to that serial killer of yours. You know more than you may think. Trust me. But first, forget about the mosquitoes. The resources within you are your emotions and instincts, not some blood sucking pests. There is no need to have actually done something to be able to put it down on paper in a believable way. Ever felt the urge to strangle one of your annoying colleagues? Now there's a good start. Channel that feeling and let your imagination run wild with it. On paper, mind you.
Another interesting aspect, and hurdle to some, is the gender of the persons they write about. Just because you're female doesn't mean you cannot tell the story through the eyes of a male protagonist (or vice versa). Keep in mind that gender is not absolute, and most importantly you will be able to dip into experiences and observations of the other sex when you want to create a male (or female) character.
Or what about creating characters who are totally different, in every conceivable way, from yourself? Just think about it – deep inside the sweetest old lady may be a ruthless murderer. Back to that serial killer, huh? People have layers, some are obvious, while others are well hidden, but the bottom line is that they are there whether you are aware of them or not. All you have to do now is take those things hidden out of the closet.
"What about the creative state? In it a man is taken out of himself. He lets down as it were a bucket into his subconscious and draws up something which is normally beyond his reach. He mixes this thing with his normal experiences, and out of the mixture he makes a work of art. It maybe be a good work of art or a bad one--we are not here examining the question of quality--but whether it is good or bad it will have been compounded in this unusual way, and he will wonder afterwards how he did it. Such seems to be the creative process. It may employ much technical ingenuity and worldly knowledge, it may profit by critical standards, but mixed up with it is this stuff from the bucket, this subconscious stuff, which is not procurable on demand. And when the process is over, when the picture or symphony or lyric or novel (or whatever it is) is complete, the artist, looking back on it, will wonder how on earth he did it. And indeed he did not do it on earth."
Edward Morgan Forster
Edward Morgan Forster
All you have to do now, apart from picking up pen and paper, is to let a bucket down into your subconscious.