As we read Psalm 139 we see we are not a mistake and that God has a plan for us. That is easy to accept unless of course God's plan seems to not coincide with ours. This is a real and often occurring circumstance. What is interesting with Psalm 139 is that as God has a plan, the Psalmist also asks God to see if there is any wicked way within him. Why ask God to search you when he already has a plan for you? This gets more complicated when he call to slay the wicked is given.
We have multiple things going on here: A Good God, the problem evil, God's sovereignty, and human choice. They all coexist simultaniously in this psalm. From our perspective the psalm means that we are no accident, but an intentional thought out by God, that God is in control and I have a choice whether to pursue him or not. I also have to deal with the fact that life is hard and not fair. We can truly hate sin and love sinners. And this plan of God is bigger than we can wrap our minds around. A big part of that plan is mercy. Because as we ask God to while out sinners, we also look to wipe out the sin in our own life.
Mercy and repentance balance the equation. BECAUSE God intentionally and purposefully made us, he does not merely wipe out all sinners. He gives a chance. Every day that life is hard and not fair is another chance God grants people to turn to him. People like your neighbors, coworkers, etc.