he almost fell into slavery and probably spread that the MC was the wrong one for sure

Well of course she'd be angry. MC was the only "Brave" that actually treats them and their village as actual people (everybody else treats them like NPCs and slaves because of the game-like mechanics). MC risked his neck into beating a dungeon with a dragon boss that is WAY WAAAAAAAY far from his level just to save the village chief's granddaughter's life. And how does he repay MC for this? He gives MC 3 gold coins (the cure or quest was worth 300 million gold) and furthermore threatens MC with keeping things quiet because he did a quest without going through the guild (because it was level-limited if he were to try and take the quest via the guild, he wouldn't be allowed to do it at all and the granddaughter would just die). Just like Shia said, the grandfather/chief tried to return a favor with evil and now he and everybody he involved in it has to pay for the repercussions since he's the one who brought legality into the discussion in the first place. The ignorant granddaughter doesn't understand the situation.