The Prompt Cupboard

Voldemort really should have investigated horcruxes a bit more

The diary had been fine, as it had only ever been owned by a teenage Tom Riddle, only a teenage Tom Riddle resided in it.

However. It should be noted that older magical artefacts, or ones that were designed to be self-sustaining, absorbed tiny bits of excess magic from the environment and those around it.

It should also be noted that horcruxes are still sentient, don't necessarily listen to the original, and tend to conform or adapt somewhat to their container in an effort to better defend themselves. This isn't always a good thing.

Slytherin's Locket, whilst originally cunning Salazars, had been owned by generations of Gaunts that had slowly deteriorated into madness, before then being stashed in Grimmauld Place for over a decade. The Tom Riddle that emerged was... uh... yeah.

The "Gaunt" ring would have been in a similar state, had the Resurrection Stone not been millenia older and created through a far higher power. Unfortunately, the Stone was geared towards luring the living into joing their loved ones in the beyond, designed to strip users of their will to live, a long lasting revenge that had gradually been forgotten by its targets. To put a piece of your soul in it for immortality? Terrible no good bad idea.

Hufflepuff's Cup was enchanted with healing in mind, strengthening the effects of potions, and was used by her descendants for that purpose for hundreds of years before finally ending up nothing more than a trophy to be paraded about. The Tom Riddle influenced by thousands of healers and millions of the sick, with generations of steadily more arrogant Hufflepuffs, probably could have revolutionised St Mungos if he wasn't an utter prick. At least he wasn't so murderous anymore.

Ravenclaw's Diadem may have originally been a marvel of enchanting said to grant intelligence to the wearer, but it was also a Goblin forged artefacts that had been buried in a highly magical forest, surrounded by wild magical creatures, in the middle of the wilderness in Romania for far FAR longer, and then dumped in a shapeshifting room stacked high with abandoned, lost, or cursed things. As a result, while the Tom Riddle in there was as "intelligent" as could be, he was also more than slightly feral. Attitude closer to the fae than the human any longer.

Now... Nagini and Harry were living breathing beings with only their own lifetime to influence anything, and the Horcrux in Nagini was already Voldemort, and only had about 3 years of influence towards the snake aspect of her malediction, which was really all that remained of the woman. He was essentially the same, only just that little built more reptilian. Perfectly in line with the original Voldemort that he was still around.

The shard in Harry However, tiny, torn at the edges and barely sentient, had been accidentally shoved unceremoniously and without much prep into a magically powerful toddler, abused for a decade, and then went through all of the utter bullshit his main self was causing from the other side of it whilst experiencing puberty again. As a result, more than a little bit protective of Harry himself. Teenage Tom Riddle was bad, Teenage-again Voldemort with a Harry complex was worse.