Description of the pattern
The use of fear to influence people's opinions toward a specific end is the goal of this pattern.
Why the pattern is good (i.e. bad)
Fear is a very big motivator. Some researchers (citation needed) have listed it a one of the biggest motivators for human behavior. Since it is such an effective motivator for action and change it can be used to influence human behavior toward specific ends. Say you wanted to motivate a nation to go to war with another nation: you could motivate people toward this end by convincing people that the other country means harm against the citizens of your country, in this way people could be convinced that the best course is to preemptively attack in order to avoid being attacked themselves.
Fear mongering is most often found in politics. One of the best known examples of fear mongering can be seen in this TV advertisement from Lyndon Johnson's 1964 presidential campaign.
Another area where fear is often used as a tactic is in advertising/marketing.
"One and a half million (of the Muslims in the United States) are radicalized to the point that they'd want to kill you." — Fox News