"The purpose of terror is to terrorize" - - V.I. Lenin.
From Strategy Page we have this article, copied more or less in entirety. That terror tactic of ISIL explained as best can be understood.
"Leadership: The Wisdom Of The Mongols"
"ISIL (Islamic State in Iraq and the Levant) has found that racing to bottom in the bad manners department can be a winning strategy. American intel analysts and Special Forces newly assigned to Iraq (many had been there before) quickly noted the ISIL was deliberately changing their tactics to maximize the ability to terrify Arab soldiers and police. Thus the frequent, self-publicized massacres of adult male opponents, especially soldiers, police and tribal militia. When captured, sometimes with promises of mercy, these men are often massacred. This is often done in gruesome ways like crucifixion or beheading. As with the mass shootings, these murders tend to be captured on video and most are uploaded to the Internet for all to see. Adding to this obvious willingness to kill ISIL also encourages a willingness to die. Thus ISIL attacks often feature lots of suicide bombers or gunmen sent into situations they are unlikely to survive. Against professional Western (or Arab) troops this sort of thing doesn’t terrorize as much as it makes it easier to kill the ISIL attackers. But since the main foe ISIL has been facing has been armed Arab amateurs (or poorly trained soldiers and police) this aggressive use of terror often works. Not just in individual battles but on a wider scale in which it encourages foes to run away even before ISIL actually attacks."
Again, understand not only terror tactics but the purposeful use of such actions combined with social media and the Internet to spread fear and panic.
NOT the actions of psychopaths or sociopaths [I might get some disagreement about that] but rather a well thought and so far successful tactic and strategy of intimidation that has worked.