Because you'll only have meaningful dialogue with someone when you use neutral language. If you were trying to convince an Islamic State zealot about faults in their reasoning they would close off their mind immediately when you use language like that.

Flippantly dismissing the Islamic State as an illogical death cult that just wants to rape and torture ignores their true aspirations and the legitimacy they draw from Islamic scripture.[1] To fight an ideology, you have to make an effort to understand it.

[1]: http://www.theatlantic.com/magazine/archive/2015/03/what-isi...