1)
Why did David compose this Mizmor?
1.
Radak: It was said with Ru'ach ha'Kodesh for the future - a generation in which Resha'im will overpower the poor.
2.
Malbim: It was against slanderers in David's time who spoke arrogantly against Hashem and falsely expounded His Torah, and spoke to their colleagues.
2)
What is "ha'Sheminis"?
1.
Rashi (from Erchin 13b): It is an eight-stringed harp.
