Liars lie, it's what they do.
Even in the face of Truth, liars lie. It was common knowledge that the Jews (scribes, Pharisees, chief priests) were looking for a way to kill Jesus. Nonetheless, the Jews and their disciples (the people who feared and followed them) lied (denied the truth) right to Jesus' (John 14:6) face – "You have a devil! Who is trying to kill you?" (John 7:20).
If liars do not lie in the face of Truth, they keep silent (John 7:26, Mark 3:1-6). Their voices are silent, but liar's actions deny the truth (John 7:30-32, Jer 7:28, Is 59:14).