God wasn't obligated to us. He was obligated to Himself. It was His justice that could not be left unsatisfied. That is where the necessity comes in. God being true to His own nature. He owed us nothing.
He was or is obligated to Himself? Based on what – higher power?
It was His justice that could not be left unsatisfied
What injustice was there to God? How can anyone cause injustice to God? Is God subject to injustice?
Does man's rejection of God "insult" Him? Does it hurt His "pride?"
Did not God design the world just as He wanted it, or was He surprized? If mankind acted the way He knew (and predestined, as you'd say) they would, where is the injustice in doing as God willed it? Or did we supercede His will?
None of this of course insults God, as God is not subject to passions. Man's disobedience only hurt man. And God, out of pure goodness and love, came down to salvage our miserable souls. Not because He needed to bring justice to Him (God is just with or without us, inspite and not because of us), but because He took pitty on ur miserable lot.