God is only under obligation IF He wanted to save some. God's justice, or nature, or essence (you get the idea :) demanded that a price be paid for man's sin in order for any to enter Heaven. Heaven is definitionally unfit for sinners, per se. Man checked his pockets and came up FAR short of the cost. Now the only choices are that God let all perish in hell (perfectly just), OR that God pay the price Himself out of His love for His elect (also perfectly just). God obviously freely chose the latter to get what He wanted, while at the same time being true to His own nature. God MUST be true to Himself, AND He had a desire. The only way to satisfy both was by our Lord Jesus becoming a sacrifice.
Being a reformed Baptist, FK, you must believe that whatever happened, including the Fall, was in God's design and that God controlled the events as they unfolded. He wrote the script, and He choregraphed it. He was not driven to do either. It was by His choice that man sinned. Did man really have a choice to change God's plan?
Amen, FK. Warfield has a great few paragraphs that respond to the "obligation of the Creator...
Even worse would have happened, indeed, than the destruction of the universe. God would have ceased to be God in a deeper sense than that he would have ceased to be the Lord and Ruler of the world. He would have ceased to be a moral being. It is an immoral act to make a thing that we cannot or will not control. The only justification for making anything is that we both can and will control it. If a man should manufacture a quantity of an unstable high-explosive in the corridors of an orphan asylum, and when the stuff went off should seek to excuse himself by saying that he could not control it, no one would count his excuse valid. What right had he to manufacture it, we should say, unless he could control it? He relieves himself of none of the responsibility for the havoc wrought, by pleading inability to control his creation. To suppose that God has made a universe-or even a single being-the control of which he renounces, is to accuse him of similar immorality. What right has he to make it, if he cannot or will not control it? It is not a moral act to perpetrate chaos. We have not only dethroned God; we have demoralized him...""...I say that it is more accurate to say that we will not admit that we are controlled. For we are controlled, whether we admit it or not. To imagine that we are not controlled is to imagine that there is no God. For when we say God, we say control. If a single creature which God has made has escaped beyond his control, at the moment that he has done so he has abolished God. A God who could or would make a creature whom he could not or would not control, is no God. The moment he should make such a creature he would, of course, abdicate his throne. The universe he had created would have ceased to be his universe; or rather it would cease to exist-for the universe is held together only by the control of God.