I don't think Dong started out evil certainly, he treated his men well and was an able commander who opposed the eunuchs, apart from one mysterious sacking in his early days there isn't much to suggest he was evil mc evil dude. It is after he arrives at the capital, sees the place in flames and everything in a mess that it gets disputable, understandably he didn't trust the old guard to clean the problems up. After all, why should he? He seized control but then, I really doubt the gentry were going to be honest and wonderful given their own track records so I hardly count that as evil.
Dong was ruthless, he forced service, he deposed an emperor and early on it could be claimed he did what he felt best for the empire. He felt Xian (who technically should have been Emperor in first place) was more able then Bian, the Han needed the best officers it could get and the habit of gentry refusing jobs hurt the administration. At the time, Dong refused to give his family excessive rank but also showed a willingness to be brutal, killing Zhang Wen to prevent a prophecy killing him. He was a ruthless man who went over the edge to cruel and at some point, turned into a corrupt, brutal tyrant out of himself, the issue for me is when did he change?
“You, are a rebellious son who abandoned his father. You are a cruel brigand who murdered his lord. How can Heaven and Earth put up with you for long? And unless you die soon, how can you face the sight of men?”