God addressing the wicked in Psalm 50 verses 17,18 & 20 says:
17 Seeing you hate instruction
And cast My words behind you?…..18 When….
20 You sit and speak against your brother;
You slander your own mother’s son.
God says that casting His words behind us, in other words despising and disobeying His words expresses itself as speaking against our brother and slandering our mother’s son.
So when we sin against others we sin against God. God’s word to us is not only to love Him but love one another. An act against another is an act against God.
We cannot separate loving God and loving others. When it comes to relationships we cannot separate the vertical & horizontal.