Fear

Fear is probably the most powerful human emotion. When it overtakes you, it is in complete control of you. That is why fear has been used throughout history in order to control people. Leaders have used our biology to their advantage in this way. We are evolutionarily programmed for self preservation at all costs, and we can be led to brutal acts just because something is threatening our existence. It does not even have to be a physical threat; it can also be a threat to our psychological identity.

Fear is something very important to understand, because it is the mind's most powerful tool against love and truth. It wants to be a stable living thing that lasts forever, and the fact that mankind has the knowledge that it will eventually die, makes it want to become as strong as possible. That is why many people's minds are like stone, and where the term "closed minded" comes from. People subconsciously shut themselves out from the world and make their minds impenetrable because they are afraid of the abyss. If you stare to long into the abyss, the abyss will stare back into you, and you are afraid of the unknown inside that abyss so you would rather create your own reality. Out of fear people create religion. Out of fear people demonize "the other", people that are different in their society and there is hatred and prejudice. People do not want their mind created reality to be destroyed, because they are afraid of what lies beyond it. 

In a way, fear is also a good thing because it pushes you to evolve. You do not want pain and suffering anymore, so you look for a way out. But actually, you usually look for the way out after all the suffering you have brought upon yourself from this fear. 

We must not let our fears control us, or else we become brutal and animalistic.