The expresion of "you didn't have to do him/her/them like that" is used in a sarcasm way. Usually, when someone gets hum1liated you say that to kinda express that "you didn't have to exp0se him that embarr4ssing way".
Do is a response used after somebody says they are going to do something. You are saying that it is something that you would also do if you were them, or just basically saying “you go do that.”
Friend 1: I’m going to pick up this girl I found at hooters
Friend 2: Do
Friend 1: I just found a dead breeze on the side of the road and I’m recharging it
Friend 2: That’s a do