If you love someone, would you willingly cause harm to them? The answer is obvious, is it?
Contained within every human being is an intuitive understanding of Love, it's something we just know. We're born with it, even if Love itself is difficult to define in any concrete terms. It doesn't come from the mind, it's not something to be intellectualized, but something to be lived in this moment. Love is dynamic and responsive, it's not stuck in the past, it's not focused upon the future. Love always has an answer in the Here & Now.
Love is the shortest route to God, and if there's any way to measure how close you are to the Divine, it's through your capacity to Love without condition. It's also the quickest way to happiness and joy, though for strange reasons the majority of people make this far more complicated that it needs to be. People search on the outside for something that was never separate from them in the first place.
Love is the greatest good, while rationalization is the greatest evil. Love doesn't need any “thou shalt nots” because Love is compassionate. Love erases separation. When you see yourself reflected within others, the selfishness that motivates people to cause harm to the other disappears. Their joy is your joy as well.
When love is absent, even “thou shalt not kill” has little meaning. Through rationalization, any thought can be twisted around and even the core morals people are taught have become very conditional. In our society, if you kill the wrong people, you're a murderer, if you kill the “right” people, you're a hero. So much for “thou shalt not kill”.