In the Vijñana Bhairava, a philosophic and practical text of the Tantric science of Kashmir Shaivism, verse 106 says, "The perception of object and subject is common to all embodied beings. But characteristic of yogis is that they are constantly aware of this relationship."
This is an extremely important point, so let's take a moment to understand what it means. Have you noticed that when you perceive people and animals around you, that you see them as "other;" as "not me?" In other words, you experience them as objects that are separate from yourself, the subject.
Because of this, you are able to treat those others differently than you'd want to be treated. Because you perceive that person over there as "not me," it's easy to think "it's not my problem" if he is suffering.
You are not even aware your mind is doing this. It just happens.
If, on the other hand, you perceived those "others" as parts of the same Self as you are, if you felt their pain when they suffered, then you'd want to treat them with great kindness and love. Wouldn't you?
This second way of perceiving is what happens with enlightened yogis. They have discovered the great spiritual Self within themselves and identify with it completely. From this vantage point, they are no longer fooled by the tendency of the mind to separate the world into subject and object. Instead, they experience the same True Self in all beings. Rather than separation, they experience unity.
This radical difference in perception results in very different thoughts and behaviors toward those "others." If others are perceived as "myself," then it is easy to "Love your neighbor as yourself," as the commandment in the Bible goes.
This is why the great enlightened beings are able to love all.
Once we come to know ourselves as the True Self, then we no longer want to be adversaries of anyone.