The detail that conservatives tend to forget is that when one party violates a contract, the other party is no longer bound by all of its terms. If you sign a contract to buy a car, and the dealer refuses to turn it over you, you aren’t “sinking to their level” by refusing to hand over your money. If you contract an employee who never shows up for work, you aren’t “repaying evil for evil” by withholding his wages. The same is true when dealing with people who are deliberately uncivil to civil people — it fundamentally changes what the rest of society owes them.
To be sure, this doesn’t mean that we must recklessly abandon civility whenever we get angry at the latest atrocious behavior from liberals. Civility is extremely valuable and is never something that should be tossed aside lightly. You need only look at the social justice left to see the consequences of doing so. Their enemies are not limited to conservatives. They rail just as hard against common sense when they melt down over beliefs that were shared by virtually everyone who ever lived until last week. They even cannibalize the very leftists who carried them to term whenever they’re triggered. Accordingly, conservatives are quite right to try and conserve valuable social structures like courtesy — they prevent all manner of chaos and suffering.