You are Tom Marvolo Riddle. You are only sixteen, but you already know time on this earth is too short. You want to live forever, rule forever. You sit in the Hogwarts library – you have practically lived here between trying to open the Chamber and finding your mother – thumbing through a thick, dusty book. Immortality is nothing but a hopeless dream to Muggles, but you – you, Tom Riddle of the orphanage—are a wizard. You are everything your weak, powerless, foolish, Muggle-loving mother wasn't. You are going to live forever!
Excitement courses through you at the thought of being young, powerful, and [though you don't think about it much] handsome for the rest of time itself! The thought of such sheer power raises goose bumps on your arm for a moment. No one will ever stand in your way!
You pour over the book that you have open in front of you. This one describes the making of a Philosopher's Stone. That would be mere child's play for you. The Stone itself, however, doesn't intrigue you at all. It will only act as means of preservation before you reach your ultimate goal. Horcruxes. You will ask Professor Slughorn about it tomorrow during class. The man adores you. He adores all those that have an aura of power surrounding them, confidence and pride. You are the center of his collection, the jewel. He thinks you can go far.
You think you can go farther.
You flip open the journal that you have enchanted to lock your secrets and plots in and write:

Philosopher's Stone
Ask Dumbledore
FLATTERY!

You close the journal and open the next book from the Restricted Section. You hadn't even needed a note; the librarian trusts you completely. You look down at what you have written and realize that this goal is completely hopeless. If Slughorn had been Flammell's partner, you would have no problem. Dumbledore, on the other hand, sees something in you that no one else seems to. It's not a good thing. You open the journal and write:

Dumbledore
What does he see?

How did he defeat Grindlewauld? [Ask Slughorn]

Kill him

You look down at the words in surprise. You hadn't consciously written them, but you realize that you want to. You want to be the person to snuff out those stupid blue eyes of his that are watching, always watching…
When you look down at the book, the word kill catches your eye.

To createa Horcrux, one must kill another in cold blood while recieting a little-known incantation. Not much can destroy a Horcrux as it contains part of a wizard's soul. To do this is to render yourself a monster.

You skim over the rest, ignoring the part about becoming a monster. Splitting your soul is a way to live forever, and within seconds you've convinced yourself that it's the only way. With your soul in seven pieces—seven being the most magical number, of course—you'd be invincible! A rush of excitement passes through you as you run back through the Restricted Section, bumping into Elladora Black along the way. She sends you a flirtatious smile, but you ignore her. You run your long, pale fingers over the bookshelves, until a small black book not unlike your journal catches your eye. You seize it and flip through it feverishly, ignoring the scrap of paper that falls out of it. An incantation is written in ink on the side of the page in a familiar curly script. You look at it in amazement. It's Dumbledore's handwriting. You stoop to pick up the note that fell out, hardly daring to breathe. In Dumbledore's flowing script reads the words: Split my soul for immortality? A warning to for the next one to have this book: This is not worth it!
You snort and toss down the note, but it surprises you that Dumbledore had once had the nerve to chase immortality as well. Perhaps you are more alike than you had thought. Maybe that's what he sees in you: A remnant of himself. You shrug and read the incantation on the side of the page. With a jolt of shock, you realize that you had been speaking these very words [in Parseltounge, no less] when the basilisk had killed Myrtle Van Dyke. The words had been written on the wall of the Chamber. Salazar Slytherin had given you, the heir of Slytherin, a path to immortality. Then you realize… if you were speaking this little-known incantation when the Mudblood died…then does that mean you have a Horcrux already?
It better not be a sink in the girl's bathroom.
You flip through the book with panic. There has got to be an explanation!

If the speaker does not specify and object to attach their soul to, it will attach itself to the next closest magical object.

You pale slightly, looking at the diary in your other hand. Could this be your first Horcrux? Disgust fills you. A Mudblood, filthy Ravenclaw, has given her life for you to live forever!
You growl in frustration and sit down. Rolphadous Lestrange, one of your friend/followers—you need to find a name for them—approaches to ask if you're all right. You wave him off. The last thing you need is this idiot following you around while you're in this state. If he's not careful, he'll wind up being your next Horcrux. He should go chase Elladora.
You open the journal, hands shaking because this is a piece of your soul that you're handling. Dumbledore had told your class a few days ago that you should eat your fears. You had snorted at him. You're brave enough—but not stupid enough—to be a Gryffindor. It is after some thought that you had come up with two: darkness and death.
You smirk and realize that you have come up with a name that will one day strike fear into the hearts of the wizarding world. Death Eaters. You scribble it down. The back of your neck prickles. Dumbledore is bent over you, reading what you've written.
You snap the book shut, half furious and half embarresed that he found you taking his advice.
He asks you about the words he saw on your pages. You answer, telling him that you couldn't think of any fears, but everyone fears death.
"Not everyone, Tom. As I have always said, death is the next great adventure. It happens to everyone, and it should not be prevented," he says.
You watch him with hatred as he walks away. You have made one Horcrux. You could easily make another. It would be so simple. Your fingers twitch on the handle of your wand. No. Not yet. Your stupid Muggle father will be the next Horcrux. Dumbledore will be your last. It will be fitting that the most powerful wizard will be your seventh, most powerful Horcrux.
You write your name. Tom Marvolo Riddle. The letters swim in your vision, rearranging into some terrifying new name. I am Lord Voldemort. You smile. Tom Riddle will not kill Dumbledore. Oh no. Lord Voldemort will be the one.
Look out old man.
There is no turning back.