You raise a very interesting point, but I think a slightly different perspective (than "God = Satan") emerges when you put those texts in a historical perspective.
1 Chronicles 21 is a rewriting of 2 Samuel 24.
The narrative logic of 2 Samuel 24 implies that "Yhwh" is not (yet) "God Almighty". We are still in a basically polytheistic (even if practically henotheistic) perspective. He cannot simply do as he pleases. To bring a plague upon Israel (why he is angry is not stated) he must resort to cunning. Enticing David to decide the census will do the trick. It's a "sin" and divine punishment will almost automatically ensue. A punishment that Yhwh could not decide alone without the trick.
This doesn't make any sense from the later monotheistic perspective of 1 Chronicles. Pure monotheism would imply that God does everything, both "good" and "evil" (cf. Isaiah 45:7). But here we are even past this stage. Practical monotheism doesn't accept anymore the idea of "God" doing "evil". Hence the narrative necessity to shift the former role of Yhwh to someone else (and here we most likely have the very first, anarthrous, use of "Satan" as a proper name). The "new Yhwh" then stays clean of the temptation and sin: his role is only sovereign justice and punishment.
Now to a couple of details:
Recently, I have been studying some material that suggests that "Satan" in the Hebrew is never defined as a rebellious angel, that only comes in Revelation (Greek) later.
Actually this character emerges gradually in Jewish literature in the pre-Christian era -- although the "enemy of God" is more often called Mastema or Belial than "Satan".
"Satan" means strictly "adversary" and is really in no way a proper name.
This is true of Zechariah 3 and Job 1--2, where the Hebrew text has the article: hstn, "the satan," which wouldn't suit a proper name. In both cases it represents a function, that of accuser or witness for the prosecution. He is a potential enemy of man (Joshua or Job) but a servant of God, defending his interests. That this role can be played by several characters is clear in the use of the plural for "satans" in 1 Enoch 20 for instance.
In 1 Chronicles, as I pointed out above, I think the perspective is already different (although some believe that the anarthrous "satan" may be construed as indefinite, "a satan" = "an adversary," rather than the proper name "Satan," but it is less convincing imo).