If we did not none of us would be here for very long, we would be jumping into fires, eating berries we find in the woods, or running into oncoming traffic. Through evolution, humans have learned to be fearful of things like that. The fear of the unknown is a fear we have developed throughout our existence. The degree of alarm varies from person to person; but we all have a certain amount of anxiety in unknown or new situations. If we feel we are not safe and secure in the new experience (for example if we lose our job and do not know how we are going to pay the bills), we will be extremely anxious, and for good reason. But even when the change is positive, like getting married, we can still feel elements of fear as we are making a significant change to our life and the consequences of that change are as yet unknown to us. To battle this fear we need to use our curiosity. An example in a social situation would be seeing a cute girl at a party and wanting to talk to her. This is a situation where we may be both fearful and curious. One may simultaneously be thinking of the implications of asking the girl out in both a fearful and curious perspective, with thoughts such as "I can't ask that girl out; she might reject me." or "I wonder what will happen if I ask that girl out.". Or in Morgan's case "This creature looks scary, it might be dangerous" compared to "I've never seen that creature before I wonder what it is.". Fear of the unknown trumps curiosity of the unknown because our instincts tell us we are in danger whether we are or not. .
Fear is simply an irrational state of mind. It is one of our strongest emotions and can drive us to do the unimaginable. When we are scared we go into a frenzied state of mind where all our decisions are based out of our fear instead of thinking things through and being rational. Hence why Morgan was so quick to kill the ant.