Because that he remembered not to show mercy - He had no compassion; he was severe, harsh, unjust, unfeeling.
But persecuted the poor and needy man - The man that was destitute of friends; that was a wanderer and a beggar. There were times in the life of David when this would be strictly and literally applicable to him.
That he might even slay the broken in heart - The man whose heart was crushed by sorrow - that he might put “the finishing stroke” to all, and send him to the grave. Whatever might have been the “feeling” which prompted to this prayer, or however difficult it may be to vindicate the psalmist’s expression of feeling, there can be no doubt as to the propriety of inflicting punishment on such a man. The sufferings invoked are none too severe to be inflicted on a man who persecutes the poor and needy, and seeks so to multiply sorrows that the man already crushed and broken in heart shall sink to the grave.