Thou hast seen it. The most emphatic contradiction that was possible to the wicked man's "He will never see it" (Psalms 10:11). God sees, notes, bears in mind, and never forgets, every act of wrong-doing that men commit, and especially acts of oppression.
For thou beholdest mischief and spite; or, perhaps, mischief and grief (see Job 6:2); i.e. the "mischief" of the oppressors, and the "grief' of the oppressed. (so Hengstenberg, Cheyne, and the' Speaker's Commentary').
Others refer both words to the feelings of the oppressed, and translate, "travail and grief." To requite it with thy hand. Again the Prayer-book Version is preferable, "to take the matter into thy hand," both for reward and requital.
The poor committeth himself unto thee. He has no other possible refuge—therefore no other reliance. Thou art the Helper of the fatherless. The word "thou" is emphatic—"Thou, and no other ( אַתָּה)."