While David’s enemies are falsely accusing him, David begs God to bring their evil to an end: 

Psa 7:9  End the evil of those who are wicked, and defend the righteous. For you look deep within the mind and heart, O righteous God.

Many times, even today, evil people try and hide their bad motives and purporting to do good.  While these types think they are fooling others, and often times are, that last bit should give us some pause.  

David says “you look deep within the mind and heart” These evil folks maybe able to fool the masses, but because God looks deep within the human mind and heart, there is no fooling God.  In other words, we may be able to hide our bad deeds from others, but there is no hiding them from God.