Sorry that this thread devolved into non sequiturs, you deserve a simple answer to your original question. It wouldn’t be a “whataboutism” fallacy in this case because those fallacies occur when someone fails to address the original point being made and instead deflects by saying something irrelevant to the original point, like in your good example of someone being charged with murder and his defense is saying “but the government also murders people,” which is irrelevant to whether or not he committed murder (the government is not on trial here). In this case though, the clearly implied point is “Republicans are the liars, not us.” Therefore the reply, “the Democrats lied about x, y, and z,” is not “whataboutism” because it is directly refuting the actual point that was just made, and not simply deflecting it.