God did not create Cain any more than he created you.
He holds creation responsible to the laws he set in place at creation. Continued existence is not creation.
His role in the continued existence of all he created was also put in place by himself then. He doesn’t have to make sure the earth is still spinning tomorrow.
You’re right about all of that ending when ever he wants. But, the continued existence part was put in place long ago by him.
Your comments are sounding like you’re thinning fee will out of the pic.
"God made us of
nothing, but by the mere act of His will He made us into
something. And the same will that brought us into existence is required to keep us in existence. Think hard about this, for in it is the primary truth about ourselves; without it we shall not know the first thing about ourselves- the
first thing.
A carpenter makes a chair. He leaves it, and the chair continues to exist. Why? Because the material he made it of preserves the shape he has given it. In other words, when the maker of a thing leaves it, it is kept in existence by the material used in its making. If God, having made us, left us, we should be kept in existence by the material used in our making-namely, nothing.
What it is made
of does not account for any being's coming into existence or remaining in existence; everything depends at every instant upon the God it is made
by. That is one reason for giving the whole power of our mind to knowing God." -
FJ Sheed, Theology for Beginners
God is as active in our creation as He was in Adam and Eve's.