Do you have evidence of this or is it merely speculation?
It's speculation based on the fact that when 2 Samuel was written the author didn't think to blame Satan, but attributed the "evil" act to YHWH's anger. It's quite obvious reading the bible that Satan is quite a late invention, but as to when and how this happened, you'll have to do your own research (unless Leolaia or Narkissos happen to grace this thread and enlighten us).
For God it was. For Satan it was not.
Do you understand FD?
Not really, no. God, out of love, conspired to murder himself. And Satan, out of hate, went along with it (although it doesn't seem from the bible that he had any part in it). Wouldn't it have been more evil to try to thwart the "murder" so that God's purpose of killing himself (or at least pretending to) in order to allow himself to forgive people of sins they hadn't commited if they believed in him would go unfulfilled? What's evil about doing exactly what God wants?