Why do the heathen rage, and the people imagine a vain thing?
Why - expressing indignant astonishment and horror.
The pagan - the Gentile nations х gowyim (H1471)], as distinguished from the Jews.
The people - Hebrew, plural х lª'umiym (H3816)]: the Hebrew singular is limited to the elect people, Israel: but in the plural it is used of the mixed multitudes.
Rage - Hebrew, 'tumultuously assemble' х raagªshuw (H7283)]: the kindred noun is translated "insurrection" in Psalms 64:2.
Imagine - Hebrew "meditate," as in Psalms 1:2: another instance of the close connection of the two psalms. The righteous "meditate" in the law of the Lord; the ungodly "meditate" a vain thing, a project which shall come to nought: their revolt against Messiah is a revolt against Yahweh Himself (for Messiah is His Representative on earth), and must therefore prove vain, as the psalm proceeds to show (cf. the parallel, Isaiah 8:8-10; also 7:14-16).