You are a prime example of WHY that lie is bad. The discussion they are trying to drag you into is an important one, and well worth having. However, by starting

You are a prime example of WHY that lie is bad. The discussion they are trying to drag you into is an important one, and well worth having. However, by starting out with a lie, they are driving you away before they can even start the conversation with you about it. A conversation where you might agree with them.