even though I think gobb 5 will clear up what happened between stingerban, we still don't have a canon backstory for them yet, so here is my opinion on what happened between them.
BTW, THIS IS COMING FROM SOMEONE WHO SHIPS THEM HEAVILY SO YES THIS IS SHIPPY AS HELL.
before bring-a-friend day, 13 and 6 were close. 6 didn't want 13 to know about his hellish form, so he didn't tell him what the intense stomach pain meant. after all, 13 was always someone who could comfort him out of it. so when bring-a-friend day came along, the core experiments broke out, including opila, stinger, and banban. the point was to escape to the surface, which was successful.
considering stinger had been dreaming and yearning for the surface for so long, he cherished the time he spent there, having fun with the kids and even parents while banban played his guitar. then, somehow, banban had some sort of trigger that caused him to become hellish, and he attacked, also triggering opila to do the same. stinger being confused and scared, he tried to round up and gather the children away but was attacked by hellish, which is what the flashback where we play as stinger flynn is, the one with hellish banban above him.
after being attacked (scar on his eye), stinger gathered up as many children as he could. when he confronted the now non-hellish banban, they got into an argument, and as much as banban said sorry, stinger had trouble forgiving. so he didn't, and he left. he hid the kids and slept for days after, not being able to find his way back to the surface. the next time he and banban would see each other was likely not long before the games take place.
keep in mind this is only my interpretation! I would love to hear some feedback or what other people think happened!!