The way of the guilty is crooked,
but the conduct of the pure is upright.
What is within us will come out in our behavior. The guilty's way will be crooked; the conduct of the pure will show in their very demeanor - standing upright with a clear conscience. The guilty must lurk about, hiding in the shadows, checking their backs, slipping across from side to side of the road, never walking down the middle. The pure walk straight and tall. They have nothing to hide and are not planning wicked deeds.
What a difference there is for the two types. One sleeps peacefully at night; the other tosses and turns in worry. One looks his neighbor in the face; the other averts his eyes. One walks with a sense of purpose and confidence; the other slinks about wary of his enemies. One is moved by the mercy of God, desiring to please him; the other despises God, wanting to escape him.
Being pure in heart is understanding that it is Christ who makes us righteous before God; it is the Holy Spirit sanctifying us; it is the mercy of God that makes it all happen. With that kind of knowledge, we will find walking straight and standing upright more natural to do.