The Woman in Number 7, by Gordon Lawrie

The woman across the road from me in number 7 is a hired killer. To my certain knowledge, she's taken out at least five individuals. She specialises in contracts on cheating husbands or wives, or the brazen husseys who break up marriages. Her methods are simple: she approaches them in broad daylight and shoots. No one ever expects a hit from a woman, do they?
 
You don't believe me, officer? It's the truth. Sure, that's my gun there with my fingerprints, there's gun residue on my hands, and my wife's blood is all over my clothes.
 
But I'm innocent, honestly.