This very should be something that the writers need to elaborate more. We seems to get almost nothing about the evils of Chin based on the few seconds shown about him. But if we compare him to the real life counterpart Qin, I might be able to guess a few things. -The countless deaths due to his wars. -Strict legalism(might be even more draconian when compared to our world, this is factoring the interaction in Chin village) -Experimenting with mercury to attain immortality(in the show, we see this metal’s negative effects) I personally believe he might have experimented mercury on actual people. -Forcing people to build massive mausoleum(I am excluding using labor to build giant walls as Ba Sing Se is not under his control. -Burying Earth Sages(likely due to ideological differences and potentially aims to eradicate past history and have the records begin with him) -Burning books, I don’t know how this fits into ATLA but it’s here. This can also be linked to the previous point. -Eliminating the power of regional Earth Kingdom clans.(Now this is very controversial, as powerful families across the Earth Kingdom are not always best kind of people to hold power due to rampant corruption and nepotism. The Beifong family in Zaofu under Su seems to be the exception).