Decency
All of nature has rules. Although unwritten and unspoken, in a primitive and savage way, right and wrong still exist there. The alpha wolf decides what is right in the pack because it possesses the physical power to exert its will. A tree may dictate what is right by exuding toxic chemicals into the soil out to its root perimeter in order to extinguish competing plants. A lion takes prey from a hyena and a hyena takes it from a leopard because that is the order of things and therefore the right thing. Instinct and physical attributes decree this form of morality in nature.
If we were just such creatures, a similar order would be the only rule over us. But we aspire to an orderly society and a higher ethic. Society, the in-your-face conglomeration of masses of people possessing tools and weapons that can inflict damage far beyond that of mere fang and claw, requires different sorts of rules and more of them. Without them we would regress to the might-makes-right order of nature. A brutal anarchy where weapons and muscle are