Did he have any outside interests that weren't connected to evil?
If I recall correctly, as a boy, he liked stealing and collecting.
Harry Potter and the Half-Blood Prince, chapter 13:
“Open it,” said Dumbledore.
Riddle took off the lid and tipped the contents onto his bed without looking at them. Harry, who had expected something much more exciting, saw a mess of small, everyday objects: a yo-yo, a silver thimble, and a tarnished mouth organ among them. Once free of the box, they stopped quivering and lay quite still upon the thin blankets.
“You will return them to their owners with your apologies,” said Dumbledore calmly, putting his wand back into his jacket. “I shall know whether it has been done. And be warned: Thieving is not tolerated at Hogwarts.”
Riddle did not look remotely abashed; he was still staring coldly and appraisingly at Dumbledore. At last he said in a colorless voice, “Yes, sir.”
Some of his main interests, apart from world domination, killing people, and eternal life were Nagini and Bellatrix.
Of course, in both cases it wasn't real love and, as for Nagini, his interest was largely motivated by the fact that it was a Horcrux.
Outside of this, he spent a lot of time studying magic and researching new spells or potions.
Even if his studies were mostly oriented to evil, Voldemort seems to have a research and a self-study attitude which is not strictly related to world domination (mastering the three Unforgivable Curses would have been enough for that), nor to his quest for eternal life (think about the silver hand spell).
Let's not forget that Dumbledore explicitly said, in CoS, that
he was probably the most brilliant student Hogwarts has ever seen.
Voldemort may also have had an interest toward puzzles. His very name was the result of an anagram, and he may have helped Quirrell in getting past the chess set.