It’s a little more complicated than what the the other replies are saying. It is true that this wouldn’t have happened had Rezyl not decorated rose with armor from a hive knight. But it was also a knight who was the mate of Xyor, a hive witch who we killed back in D1. Xyor took the opportunity. It wasn’t just that the thorn was whispering. It was more Xyor using hive magic to whisper through it, like a voodoo walkie talkie. However we don’t know how the death of Xyor affected thorns corruptive capabilities, but it likely didn’t affect it much as the whole thing was complete by that point

The bones grafted to the gun begin to whisper to you, plant thoughts in your head with auditory hallucinations only you can perceive. There is also some evidence to suggest that the gun’s very presence (at least Yor’s original Thorn) was so far gone that it’s very presence leached light (in the Lumina quest, the text compares it to the feeing of losing your Light in the Red War), meaning that it’s possible it renders the wielded incapable of harnessing Light in later stages of corruption.

Feel like you've got to use it to get corrupted by it, although simply being near it definitely has some effects. Like the One Ring from LOTR. A lot of different characters were near it and temporarily fell under its influence, but the only way to be completely corrupted by it was to wear it a lot

It’s Hive voodoo magic. EDIT: Weapons of Sorrow are basically human weapons that merge with Hive magic/tech/whatever they have. The Corruption is basically a manifestation from said Hive magic. Therefore Thorn fires rounds with the Evil stuff.