After agreeing to a legal transaction, following a bout of haggling, the PC attacked the merchant, broke the agreed upon promise to pay 80, and then effectively stole from the unconscious shopkeeper by shorting him 30 gp. This was all done for personal gain and was based upon a "might makes right" abuse of power because the PC figured they could get away with assault the merchant with no repercussions.

Using your power to break the law, abuse those who are weaker, and doing it all for personal amusement and gain is pretty much textbook Chaotic Evil. Since the PC didn't kill the merchant or burn down the shop, I'd be willing to gauge it as CN since it's not full murderhobo