oh 100%. i have been reading the raws (looking at the pictures that is) so can't say if that's confirmed but it's inferred enough that the uncle recognised HG budding masochistic tendencies (which is really sad bcs it's often a sign of child abuse/neglect) and is encouraging them. whether for his enjoyment or some other ploy, it's hard to say. maybe both.
It isn't "masochistic" it's "psychopathic" and studies show that psychopaths at a young age tend to develop slower like Heonjae did. Being a masochist is being someone who likes controlled pain in a smexual manner, and has absolutely nothing to do with psychopathic tendencies like what serial killers do
I reckon their Uncle got Heonjae a cat or a dog, and Heonjae eventually started to carve its skin up and peel bits of its flesh off with a knife as a form of "love" because of that chapter ages ago where he said he'd love to do that to Myeonggi... Just a guess tho, I haven't read the raws