In criminal law, we only consider NORMAL people's behavior,

not behaviors of abnormal people.

For example, you cannot kill another person simply because you think he is ugly. That is your abnormal behavior, not an average person's behavior. The society would not tolerate that kind of abnormal behavior. This is called social norm.

An average, normal person also becomes nervious under certain circaum. However, an average, normal person will not shoot at others recklessly when becoming nervous. 

Think about it: if this society tolerates all nervous people to shoot others dead at will, isn't it a hell??