It's human nature to mock the things we fear. Comedy helps to take the sting out of uncomfortable subjects such as war and death. Come Halloween time, people often follow along that same pattern of thinking when it comes to costumes. "Ripped from the headlines" costumes are also always popular, and of course some people just like to pick the most controversial garb as possible simply for the shock value.
So it's not surprising that many people this year as dressing up as ISIS terrorists.
Some go for authenticity and realism, even adding guns into the mix. (Note: Definitely NOT recommended for public events.)
Some are even choosing ISIS as a theme for couples costumes, which somehow makes the twisted idea even more bizarre.