Just like in physical fights, online arguments can have a knockout blow. That one line so devastating, and with so much outside support, there is no coming back from it. Losers would be better off shutting up or deleting their account. 

You probably know about the Twitter "menswear guy," and he's taken it upon himself to deliver some truly scathing roasts recently. One example came against a man claiming that men and women do everything, including dressing, with the subconscious desire to help them find a mate. The menswear guy pointed out that the tweeter's chosen career of "aquatic plant scientist" obviously wasn't made with that in mind. 

Read that exchange and 20 other scathing comebacks that left people contemplating account deletion.