We were sixteen when we killed a man. Not just any man, not just for shits and giggles. He'd hurt you, this man. He'd hurt you, but you would never say how or why, and even though I'd try to guess I knew I would never get it right. And even if I did, you wouldn't confirm it. Wouldn't or couldn't. You'd hidden it away, deep down in the dark, in the boxes where you kept everything else that you weren't allowed to look at or remember.

I was consumed by anger and rage and hate and confusion. I sought revenge for you, because I'd failed to protect you, and that's not what friends do, right? But you just tagged along, because you were always tagging along. Because we were friends. And friends don't like it when their friends are hurt and angry. That's how things work.

We went after him one night. Waited by the drive outside his house until he came back. There was a knife, a crappy kitchen one with a blade that wobbled like a loose tooth. But it wasn't our moment yet – he'd returned with that blonde, surgery-stretched young girl swaying on his arm. She had the brightest red lipstick on her mouth, a little smudged at the corners, and he was wearing it on his mouth as well, like two little girls trying to look alike. He opened the door with one hand, because the other was stopping her from falling over on those ridiculous silvery stiletto heels she was wearing.

The moon was large and bleeding in the black of the sky. We waited there all night, because goddamnit, we weren't going home now, not when we'd got everything ready, when it was all so beautifully prepared. Home wasn't the greatest place to be right now, anyway.

Most murders are committed in the dark. At least, that's what the movies would like us to think. But contrary to popular belief, it's the other way around. Darkness always makes things scarier – it's better shock value. There's more room for fear. More than that, darkness is where the monsters live, where the demons come out to play and to hide in the shadowy corners. We should know. When all's said and done, we belong to the dark.

That night, though, the dark was not on our side. So it happened in daylight instead.

His house was set quite far back from the road, at the end of a long drive framed by trees. If we wanted to set the scene a little more, we'd say what kind of trees they were – sycamore, maple, poplar – but you couldn't remember, and I couldn't give a shit. Who cares what kind of trees they were? But we do remember that it was raining. We remember that oh so vividly. Because when I cut his throat that morning, the blood flowed and mingled with the puddles and turned to a light blossom-pink. It was kind of pretty, the colour you'd paint a sunset in. And we left the body there with the rain slowly bleaching it of life, skin turning the icy grey-blue of a Titanic victim. We left and we walked away, hand in hand, together. Just like always.

You were scared, then. I felt your heart beating fast as a rabbit's, and I held your hand, soothed you, kept you safe. You were always the soft one, the one who was afraid of the dark (or of the people in the dark, I was never quite sure which) and I was the hard one. I had to be. I stop you from thinking about things, you told me once; when you're around me you're braver, stronger, louder, different. Without me, you're nothing. A wisp. A phantasm. There are a hundred things you've tried to chase away, the things you won't remember and can't even let yourself think about, because that's when the birds scream and the worms crawl in and the worms crawl out and somewhere in your mind it's always raining a slow and endless drizzle.

They found out, of course. We didn't mean them to, but they did. There were fingerprints there, on the man's clothes and his face; they dusted him all over with black powder to see them more easily. He'd struggled, tried to fight back, and scratched my face. My blood was under his nails. So was yours. He condemned us even though he was dead, and I knew I could never forgive him for that.

Your parents cried when they found out. Sobbed, like small children afraid of being alone again. It was heartbreaking, really. Their baby had grown up to be a monster. It was me who had killed him, when push came to shove, but that didn't change anything. You were still a monster. A crawling demon in the dark. Something to fear. You tried, over and over you tried, to tell them that it hadn't been you; that you hadn't really done anything except trip him up, that it was me who'd done all the important work, that you'd just tagged along because that was what friends did. But it's rude to interrupt people in nice clean suits.

It's okay, though. Wherever you go, whatever happens to you from now on, I'll be here. I'll protect you. If anybody tries to hurt you ever again, you'll know who to turn to. Because I am you and you are me, and if I leave you are nothing. A soft shapeless thing, with no substance and no soul and no heart. Half of a whole, and broken beyond repair. That's all you are. All you ever were.

You can't forget a history.

Everyone dies, leaves or just forgets to stay, you told me once, with a voice dry and crisp as autumn leaves. Well, I'm not going anywhere. It's us against the world, remember? And you can't leave me either, because that's not what friends do. So stay with me, just for one more night. I'll hold your hand, keep you safe from the other demons in the dark that might prey on us. I'll protect you. I will always protect you.

You and me. Together.

And that's how it's always going to be.