Many scholars believe the Nimrod character is a collective amalgam of the early emperors of Akkad, particularly Sargon the Great and his grandson and third successor, Naram-Sin, under whom the Empire reached its greatest extent, and those of the Babylonian, Assyrian, and Neo-Assyrian Empires which succeeded it.
It is worth nothing that the story of Moses closely parallels the birth-legend of Sargon. Both were placed in reed baskets, set upon a river, and rescued by their respective royal households. Sargon became cup-bearer (a kind of regent or Prime Minister) to King Ur-Zababa of Kish, but overthrew him when Ur-Zababa conspired to kill him with the Sumerian king (Lugal) Zagesi. He then conquered Zagesi's confederation of city-states, and centralized control in Akkad.
In other words, Akkad is considered by the Bible and historians alike to be the world's first empire-proper, and Sargon the first emperor. See
Genesis 10:8, 10-11 --
Cush fathered Nimrod; he was the first on earth to be a mighty man... the beginning of his kingdom was Babel [Babylon], Erech [Uruk], Accad [Akkad], and Calneh, in the land of Shinar [Sumer]... from that land he went into Assyria... This matches almost perfectly with what history knows of Sargon the Great: he conquered Sumer and established an empire from Akkad that later split and became Babylon and Assyria.
Cush/Kish is probably a coincidence, but an interesting one -- Kish did 'father' Sargon in a sense, because he did not know who his father was, and his mother cast him away, so he became a "Kishite."
It is no surprise that even many generations later, the Hebrews viewed Sargon as a "mighty man" -- the first, even -- and a "hunter" (literally* and perhaps figuratively) before the LORD -- a great conquerer, seemingly god-like and ravenous against his enemies -- the same goes for Naram-Sin, from whom the name 'Nimrod' is probably derived.
The Neo-Assyrian Empire under Tiglath-Pileser III invaded Israel, demanded tribute from Ahaz of Judah, and exiled the Israelites living in Samaria to Assyria where they were enslaved, killed, and/or forced to adopt paganism. Sargon II ended the kingdom of Israel and exiled its inhabitants, now known as the "ten lost tribes." Sennacherib besieged Judah; Ashurbanipal subjugated it. These were, perhaps, the "hunters" -- *
ritual lion hunts were an important Assyrian tradition -- Ashurbanipal was particularly known for his elaborate Nineveh palace reliefs of the royal hunt.
Nimrod's significance in the Bible is this: the Israelites viewed the Neo-Assyrians as the supreme enemies of God, idolaters who defiled Israel and forced their strange deities on God's people through conquest and subjugation -- hunters, literally and figuratively -- descendants of the "first mighty man on earth." Sargon's legacy is portrayed as a profane imitation of the power that belongs to God alone (i.e., he was seemingly invincible, an almighty ruler, etc.), and yet he was also passively revered as "a mighty man before the LORD" -- a begrudging and mildly sarcastic phrase.
Nimrod represents all of the negative, profane, and blasphemous tendencies of Sargon's successors, but he also represents the latent pride the Hebrews must have had in him as the "founding father" of Semitic dominance over the region.
The character of Nimrod is neither entirely bad, nor at all particularly good, but rather cautionary and somewhat mournful, as if to say "if only our people had served the LORD and not strange idols, we would be mighty on the earth and not scattered and lost."