Psalm 101 Verse 1-4 are David's expression of his will, his desires:
To sing of loving-kindness and justice.
To sing praises to The Lord.
To walk carefully and blamelessly.
To have integrity.
To have nothing worthless before him. No idols.
And to know no evil.
But, David thoroughly condemns those who:
Secretly slander their neighbor.
Are arrogant and haughty.
Practice deceit.
Speak lies.
David surrounds himself with those who are faithful.  He trusts those who have integrity.
He has determined to clean house, so to speak.  Only those who are faithful to God will minister to him.  He also determines to destroy the wicked.

David clearly knew God's heart, for we are told that he was a man "after God's heart".  He clearly knew God as Lord, Who is just and merciful.  But in this Psalm, he seems to lean more to the justice part.  Does there come a time when mercy has done all it can and judgement comes?  Are we to make that determination?  David looks to himself first in this psalm.  He examines his own heart.  He looks to The Lord.  Only then does he speak out against those who do wrong.  He doesn't ignore or condone sinful behavior. These wicked ones are not asking for mercy, they are arrogant in their sinfulness.  They tried to stay in his house and act this way.  This he would not put up with!  Jesus warns of wolves that come in sheep's clothing, infiltrating the flock.  We are warned to be careful, to be alert. To know them by their fruit. ( Matthew 7:15-16)  This is what David did. This is NOT being judgemental.  It is be upright before God.
Father, speak Your truth to my heart on this matter. Give Your wisdom I pray. Amen