A consistent theme throughout the Bible is love. Love for God, and love for one's neighbour. What's interesting is that the two are very closely linked. You cannot love God without loving your neighbour, and to sin against another is to sin against God.
This explains David's strange words in Psalm 51:4, when he says "Against You, You only, have I sinned, And done this evil in Your sight". It can hardly be said that David sinned only against God. He slept with a man's wife, resulting in her pregnancy, then had him killed so he could marry her. That's three human beings he had sinned against aside from sinning against God. And of course, as a king, he ultimately disgraced his country in doing so, thus sinning against thousands of people. And against himself. It seems that really, there's no one David didn't sin against.
But still he declared "Against You, You only, have I sinned, And done this evil in Your sight". This is because all sin, be it a personal sin or a sin against another living being, is sin against God. Thus, love for God and love for others cannot be separated.