But when Gamzee said that Terezi was hurting him, wasn't it just a distraction so he could stab and harm her right after that? Terezi was rightfully fighting against him. (Or, at least, that was my interpretation of what happened.)

Yep, as soon as he said that Terezi dropped her weapon, started crying and apologizing. Then Gamzee proceeded to beat the ever-living shit out of her.