The ISIS fighters are more fierce than the Iraqis because a lot of them are drugged up. They take coke, meth, and whatever to hype them up and not care if they're killed and not care about human life. They also intimidate males to fight for them under threat of killing their families, and they know that those aren't empty threats.

http://www.cnn.com/2014/10/28/world/meast/syria-isis-prisoners-watson/

http://www.dailymail.co.uk/news/article-...ost-morale.html

http://nypost.com/2015/01/07/are-isis-fighters-fueled-by-piles-of-cocaine/