Intolerance lies at the very heart of evil. Not the intolerance that results from a threat or danger. Nor that which arises from a negative experience. Simply the intolerance of another being who dares to exist, who dares to diminish the space in the universe left for you. Intolerance without cause.
It's something so deep within us because every human being secretly desires the entire universe to be theirs. The only way to combat that is to cultivate compassion without cause—to care for others simply because they exist.