He had been in the second floor girls' bathroom, opening the Chamber of Secrets to release the Basilisk on the sheep of the school (namely, the mudbloods and the blood traitors) when a shout came from one of the stalls, demanding he leave. The Basilisk acted on its own imperative, slithering into view of the stall and affixing its gaze onto the occupant. A body slumped out, dead.
The dead eyes of Moaning Myrtle stared at him.
Oh, that's what everybody called her, anyway. Tom himself had never bothered learning her last name. And he had killed her.
He paced, thinking of what to do. He knew that the murder would not be traced back to him, as nobody knew of his connection to the Basilisk. Probably. His mind began working away at the problem, anyway; some (the Transfiguration professor came foremost to mind) had suspected him no matter their lack of proof, and so he had to distance himself as much as possible anyway.
Hagrid...
He remembered the oaf of a giant was keeping an acromantula specimen which called itself Aragog. While Tom appreciated the beast by itself (though he was more of a snake man), he despised how Hagrid treated it like a dear friend. Magical creatures, especially one so Dark, ought to be treated with the respect they deserved -- letting your guard down a moment could be fatal, or often worse.
He thought he could use Aragog against Hagrid -- reveal the acromantula's presence, claim that it had Petrified all the students (and killed Myrtle) over the past year, and get Hagrid thrown into Azkaban.
Sure of his plan, he almost left his quarters to find a teacher, when a thought tugged at him. He remembered Professor Slughorn had told him once of a way a wizard could extend his life indefinitely. In the library, he had discovered the exact means in 'Secrets of the Darkest Art', and had memorized the instructions on the spot. Now, it seemed, would be the opportune time to see if he could craft a horcrux of his own.
He looked around the room, wondering what he could use for the vessel of his soul fragment. At last, his eyes alighted upon his diary. It took him nearly an hour to prepare the ritual.
Finally, he began. He muttered a series of complex spells which were meant to bind the soul fragment to the diary, and waited expectantly.
The diary began to glow...black. Rather, light seemed to somehow slip off of it, like water did a duck's feathers. Tom allowed the smallest malignant smile to play across his face...
The book suddenly burst into flames, turning into ashes within moments. The smile was shattered as his mouth fell open in utter shock at what had just happened.
He stood and furiously kicked at the cinders which once held his record of the past six and a half years. He stifled an irrational burst of hatred towards Professor Slughorn -- the old idiot could hardly know that the horcrux charm didn't work.
He forced himself to breathe slowly and draw in his rage until it could be constructive. Right now, he had to find a teacher and turn Hagrid in.
After walking the halls of Hogwarts for several minutes, he met one. The last one he was hoping to see. Albus Dumbledore, Professor of Transfiguration.
"Tom, I am glad to have found you," the elder wizard said, approaching him with urgency. "The Heir of Slytherin has struck again."
Tom feigned shock. "Someone's been murdered?"
Dumbledore looked startled at this. "No, Tom. A Ravenclaw girl was Petrified in one of the bathrooms."