Touching a grave made a person ceremonially defiled (Num. 19:16), so graves were whitewashed to warn people. An unmarked grave was a hidden source of ritual impurity.[i]
To come in contact with the religious leaders was to be defiled by something dead. The fraud was that the graves were unmarked. You could not tell by looking at them that they were spiritually dead. So the deadly influence of the religious leaders went undetected.
They had a defiling, negative influence on people but it was masked by their righteous image. Sometimes people look as if they are spiritual and godly, but they can defile you without you knowing it.