It's about our heart

1 John 3:15  "Anyone who hates his brother is a murderer, and you know that no murderer has eternal life in Him."

John is speaking to us in this passage about how your actions and your attitude toward sin are evidence of Christ being in your heart and in your life.  He argues that fulfilling the command to love always results in actions that show the presence of love in our hearts.  If we say we love someone but do not express it in sacrificial service, do we truly love them?  Likewise, he argues that one who continues to willfully sin against Christ does not know what it truly means to have Christ in their heart.

The verse above speaks to God's lack of a ranking system for sin.  We read this verse and might think that putting hatred and murder in the same category is a little extreme. In our minds, we can harbor hatred against someone, but as long as we don't act on what we feel, we haven't committed a sin against them.  But God hates all sin, and doesn't differentiate between specific types of sin.  Looking at a woman with lust is the same in God's eyes as committing adultery. 

Not acting on what is in own hearts is no grounds for self-congratulations.  What is in your heart, good or bad, is the ultimate issue.  God wants our heart, not just our outward obedience.