The aggressor sets the rules.
So you have two choices: either get nasty back or sit on your ass and watch them take over.
You choose.
There’s an old saying: all evil needs to prevail is for good men to do nothing.
There's a difference between "getting nasty back" and cheating to beat the Leftists, and I'll admit that conservatives haven't been tough on the Left by calling out every violation. Obama's likely illegitimacy is exhibit one on this.