[gretagretagreta]

If A criticizes B's behavior on moral grounds, with the hope of, say, "making the world a better place", then it would be logical mistake to respond to the criticism by pointing to bad behavior on A's own part.

If, however, A's criticism of B is not an attempt to "make the world a better place" but to prove one's own relative moral superiority, then not only is it highly pertinent to point out A's own moral failings, but also there need be nothing relativistic about it, since ex hypothesi the conversation no longer concerns the morality of this or that action but only whether one party has a legitimate claim to feel superior to the other.

It's not always easy to tell whether one is in the one or the other situation.