You are legally and morally obligated to keep your promises but not your threats. If I, as a landlord, threaten my tenant that I will increase her rent if she does not mow the lawn and if I, out of my own sovereign will, for any reason, do not actualize my threat, then I am not a liar nor am I guilty of a breach of promise. Ditto for God. God can threaten all He wants but if he commutes his sentence out of His sovereign will then we cannot accuse him of falsehood or perfidy, not can be get angry at Him (as Jonah did).
Another way to look at Hell is that it is our ground state, our state without God's inflowing grace. In this sense, all of us deserve to be in Hell and Hell is real as where we SHOULD go minus the grace of God. But grace is a journey as well as a moment of light. Even if I deserve to go to hell today, today is just a slice of time in my journey. God can lead me and everyone else on the road to Damascus. We might think we're going to Damascus to commit a specific atrocious act, but God might be leading us to Damascus to do other things for Him. Thus our destination, Damascus, which was a hell of egregious perfidy is transformed into a heaven of godliness - all by the Light of God.