To be God.

It sounds very interesting and I can't wait to see how it plays out! It left me with so many questions!

It is like the work of a Devil, running riot in your head. It is like watching ten thousand knives fall from the sky, landing anywhere in the World

It is a very dramatic description but hard to understand.

I also noticed that Catherine's father never writes his name. Does he think his name is sacred and shouldn't be written or did it just slip his mind?

Hey:)

Wow, this is so creepy. But GOOD. A couple of tips for you though:
It was hard for me to follow what exactly you (or the writer of the letter) was meaning. Especially this line:

It is like the work of a Devil, running riot in your head. It is like watching ten thousand knives fall from the sky, landing anywhere in the World.

I didn't really understand that. Especially the running riot bit.
Also, I found that you were being very descriptive, which isn't typical of a serial killer. Less is more. I would rather have my imagination fill in the blanks (to some extent) rather than having all the language and words in there. Of course, he is writing to a loved one, but I still think there's a lot of extra words.
But after some improvements, this can be something even better. Keep up the good work.
Polina x

Chilling. Especially this:

I tell you, you are now God

One of the problems I have is the way it is written. The language in which this letter is written is definitely too flowery. You're describing too much. He didn't write a short story, he wrote a letter. The voice, though appropriately sinister, seems too proper; more proper than I would imagine a demented serial killer being.

Nitpicks:

Here’s your piece with edited punctuation and spelling (there may be things I missed):

Dear Catherine,
To play God is a dangerous act. It is like the work of a Devil, running riot in your head. It is like watching ten thousand knives fall from the sky, landing anywhere in the World. To be God is a wonderful thing; it is a gift. Your mother did not appreciate the gift that was brought upon myself. That is why I took a knife to her throat, and I let the cold metal sink into her soft flesh, the red liquid oozing out faster than I could control. Exhilarating really, to watch a woman, so naïve, she will walk willingly into the hands of the man who will end her life. We, as Gods of our own, can construct all the reasons in the World as to we take to killing, but everybody knows, you can’t see the light if there is no end to the tunnel.
You were 7 years old when I started my new role as God; your mother was my very first. My last was a man who you will never really know, he was the man that broke your mother’s heart in two, and by the time you read this letter, he will be the dead father of a young woman who has just finished a letter. What is in between these 2 deaths lie secrets, that even you, may not want to discover.
Catherine, you are my last hope. As my only request as a man sentenced to death, I give you a name and an address, you know what to do. One last piece of advice, I have known you for 19 years, I know that for the last 12 of those years, you knew what I had done, and for that, I tell you, you are now God. We are all selfish people, and one way or another, we must seek happiness.
Goodbye Catherine.

It is like the work of a Devil, running riot in your head. It is like watching ten thousand knives fall from the sky, landing anywhere in the World.

This part doesn't make a whole lot of sense to me. I feel like you're just throwing in descriptions. It's not very flattering to your piece.

Your mother did not appreciate the gift that was brought upon myself. That is why I took a knife to her throat, and I let the cold metal sink into her soft flesh, the red liquid oozing out faster than I could control.

The description at the end about killing her is over the top. Like I said, way too flowery. I would honestly just keep it as: "Your mother did not appreciate the gift." Remember, when writing about serial killers, the less you write, the creepier it is, because it leaves the reader to go to their imagination.

Exhilarating really, to watch a woman, so naive, she will walk willingly into the hands of the man who will end her life.

This should read as: Exhilarating really, to watch a woman--so naïve--walk willingly into the hands of the man that will end her life.

We, as Gods of our own, can construct all the reasons in the World as to we take to killing, but everybody knows, you can’t see the light if there is no end to the tunnel.

This part makes absolutely no sense to me. The two parts of this sentence are completely unrelated, it seems. It would be like me saying: "We are all Nazis, and everybody knows you can slip on a wet tile floor if you're not cautious." I would change this.

You were 7 years old when I started my new role as God, your mother was my very first. My last was a man who you will never really know, he was the man that broke your mother’s heart in two, and by the time you read this letter, he will be the dead father of a young woman who has just finished a letter. What is in between these 2 deaths lie secrets, that even you, may not want to discover.

This is written really bizarrely. I don't really know what he's talking about. I would rewrite this to make it more clear.

Other than those nitpicks, this was a very nice start. I'm excited to see where you'll take this.

Keep writing!

H.