Praise be to Allah.
Al-Shaheed is the One Who is honest and trustworthy in His testimony. It was said He is the One from Whose knowledge nothing is missing. Al-Shaheed is the One Who is present and witnessing. He is also the One who will testify against His creation on the Day of Resurrection. He points to His Oneness (Tawheed) in all that He has created.
Al-Shaheed is al-Khabeer (the All-Aware), al-‘Aleem (All-Knowing). It all comes back to His knowledge; the first name refers to that which is apparent, the second refers to that which is hidden, and the third refers to both.
Because nothing is hidden from Allaah, He is both Shaahid and Shaheed (Witness), i.e., He knows their reality like one who sees and witnesses them, because nothing at all is hidden from Him.
And He will be the Witness for the oppressed who have no other witness or supporter against the aggressors and oppressors who have no one to stop them in this world from abusing the rights of the oppressed. So He will restore their rights and avenge them.
Allaah is the One Who is watching over all things. He hears all voices, loud and silent alike. He sees all that exists, great and small alike. His knowledge encompasses all things, and He is the One Who will bear witness for and against His slaves with regard to what they do.