Islam does not promote killing innocent people especially like women, children, elderly; this is in reference to comment regarding Brussels. The Prophet Mohammad (PBUH) issued strict orders forbidding the killing of non-combatants, destruction of cities and populations, resort to rape and looting of civilian population, and killing of the enemy by starvation/thirst or any form of torture.
Can wage war against infidels in defense of self when attacked and if they seek peace, then it's ok for Muslims to negotiate terms of agreement for peace.
The problem becomes when ISIS pick and choose clauses from Quran/Koran and ignore the other ones for their benefit. They can fight against infidel armies or government subsidiaries, but not against innocent people.
From Quran: "And fight in the way of God with those who fight with you, but aggress not God loves not the aggressors. And slay them wherever you come upon them, and expel them from where they expelled you; persecution is more grievous than slaying"
Jihad in Islam means 'struggle', not just war. It is struggle against corruption, injustice, aggression, and even against self if one is trying to correct bad habits, i.e. lying, cheating, adultery, not praying, etc.