I think he was on that spite, anger, and sadism kind of drive. Heonjae said that Jingi likes to torture them and enjoys it when he sees them struggling. Also Jingi kinda just does it to vent anger and stress, bcuz he always says "I'll take my stress out on Myeong-il".
So in the first chapters, he said he was feeling pretty sh*tty because he was reminded of the first day he came to prison, and was also pissed because he couldn't get a phone or his correspondence that time, so he was venting it through torturing that fat inmate until he bled. (It's Heonjae I'm wondering about, because he doesn't do it to vent, so I don't know why he used to do it with those fugly inmates).
anyone remember when Jingi was fucking that big fat buff man at the beginning? that's what I call no standards. how did bro even get it up for him. wtf was he on fr