Humans are hardwired to interpret certain physical features as cute, triggering our nurturing instincts, regardless of whether the animal in question is safe to interact with. I'd personally love to give scritches and belly rubs to all sorts of floofy animals that would at best strongly object to me intruding on their personal space and at worst rip my face off.
Do you have any dangerous critters that evoke the same feelings? Are there other sophonts besides humans who have a different schema for cuteness?
The Lonely Galaxy has tree dwellers (or tree-dwellers), which are nonsapient congeners to the yinrih. They're visually identical to their sapient kin as far as humans are concerned.
Adult yinrih have a very strong sense of personal space. Their wild ancestors did not engage in allogrooming, since their jungle home had plenty of bristly plants that they could rub against or wallow on to get rid of mats and bugs. Gestures that involve physical contact are vanishingly rare among unrelated adults, including between parents. So when humans try to go in for a snoot boop or belly rub they're likely to be met with an assault charge.
However, pups are more physically affectionate. Siblings will thump each other across the back with their tails, sires and dams will "kiss" their pups (touching the wet tip of the nose to the muzzle, top of the head, or back of an ear and quickly exhaling), and parents will often intertwine their tail with their pup's to give comfort. As pups get older, they start rejecting physical affection from parents and litter mates, though they may resume them to some extent as adults.
Tree dwellers display similar developmental stages, with kits and pups actively seeking physical interaction from sires, dams, and litter mates, but rejecting (sometimes violently) this affection as they near adulthood.
Tree dwellers are almost as long-lived as yinrih, with more or less the same ~50ish years to reach adulthood, with a good 30 or so of those years where the pups are cuddly. So you can probably see where this is going. A human illegally buys a tree dweller pup, which will be all cute and cuddly for potentially the rest of the owner's life depending on the ages of the pup and owner, and by the time that pup grows into a violent adult, the owner has died and this critter who will live longer than the Western Roman Empire is now someone else's problem.
It must be bigger than a normal butterfly if it's a parasitoid of humans.
Define "normal butterfly". This one has a wingspan of about 15cm, so it is rather large. But size does not matter here. The power of the nerve agent does.