After a disaster, there are two people left in the world, A and B. Each fully loves the other, and each fully loves herself. However, they hear noises in the woods occasionally, and come to form a justified false belief that there is a human being in the woods. In fact, there isn't—there is just wind. Now, A comes to love the alleged person, exhibiting the love by leaving food out. On the other hand, B comes to hate the alleged person, exhibiting the hatred by setting traps.
Both A and B love each human being. Yet, clearly, B is doing something wrong. So, it seems, loving each human being is not sufficient for morality.
Yet the New Testament suggests that it is. It seems part of the message of the New Testament that love of God and neighbor fulfills the moral law. And the First Letter of John suggests that one loves God to the extent that one loves one's neighbor.
I think there are two moves here. One is to take the claims in 1 John as applying to typical cases. Typically, one loves God to the extent that one loves one's neighbor. Cases like that of B are too out of the way to be what the author of the text was talking about. And then one can argue that B by failing to love someone that he takes to be a creature of God is failing to love God. So on this view, the foundation of Christian ethics is not just love for neighbor, but either both love for God and love for neighbor, or just love for God, though typically love for neighbor is indicative of this.
The second move would be that to love one's neighbor requires not just that one love each individual neighbor, but that one have a disposition to love all possible people, should they be actual. This seems right to me, actually. For another kind of case would be this. Suppose that the world is down to two people, and they are virulent racists, but they are both of each other's preferred race. They love every human being who exists. But they don't have a disposition to love all possible people.