Confused about the Strogg

Thats a good question. I think that some of the enemies in Quake II arn't human. The Engineires (i can't spell it!!) are giant brains in floating barrels, The Tanks look like they are 100% machine. The Icarus don't look human at least. But this is just my opinion.

Berserkers, Gladiators, Gunners, Iron Maiden, Scientists (Q4), Tanks, Enforcers (Q2), Icaruses (Q2), Medics (Q2) are all "made" out of non-humans and probably from the original Strogg aliens.

Stroggified Marines, Tactical Transfers (Q4) and Guards (Q2) are "made" out of Earth humans. The differences are more apparent in Q2 where all the strogg-cyborgs shared some facial features and were a lot taller than the human-cyborgs.

http://www.planetquake.com/quake2/q2guide/enemies.shtml
http://www.planetquake.com/quake4/sp/guide_s3p1.shtml

I recall while playing that there was a reference to the Strogg being a disused army of super soldiers created by another race. I assume that after their creators abandoned them they were able to operate independently and act as any other race would; to further the existence of its species.

This wikipedia article offers some excellent information on the Strogg: http://en.wikipedia.org/wiki/Strogg

The "real" Strogg have never been shown, at least not expressly.

Quake 2 is the first instance of the Strogg, where they attack Earth intent on taking it over, not because of wanting to kill humans, but because they see it as a tactical location that can be used to launch invasions on many other solar systems.

The Strogg were a biological species from an industrialized society who added machines to their bodies, much like we do today (replacement limbs, pacemakers, etc). As their technology advanced and their world was destroyed by their own industries they attacked other worlds to secure more and more resources. As time went on they became more and more dependent on technology, and the fusing of Strogg and machine became commonplace, especially since modified soldiers proved to fight better than non-modified soldiers. Using other species for additional soldiers became an easy way to increase the sizes of their armies, and continue to fight for more resources.

There has been no explanation of Strogg "civilian" life. We don't even know if there are any civilians. From the games all we've seen are Strogg who fight and strogg who maintain the equipment they use to survive. Playing through Quake 4 you see a wide range of creatures used not only as soldiers, but also one creature which proves to be the lynch pin in their "food supply", just another example of how they make use of different species.

Keep in mind, even species that look human may not be. It is entirely possible that the Star Trek view of Aliens all having the same basic structure as humans is possible. Lots of the creatures we see in Quake 2 and 4 are human looking, but WAY too large to actually BE humans.