What IS needed to kill, is the will to kill....
Yes. That's an important observation. And the will to kill can be enflamed by various motivations. One, and primarily: religious perspectives.
I think its evident that the Christian crusades were undertaken to advance the goals of those who embraced the Christian faith. Similarly, the expansion of the Ottoman Empire was to further the spread of Islam. Now, I would certainly concede that even if there were no religions, ever, there would still be wars. Any number of wars were fought over wealth, land, and power. However, absent the religious perspectives pulsing away, there would be one less reason to go to war and thus reduce the human carnage MASSIVELY.
But historically speaking, today's examples serve as well. The conflict in the Middle East is about land and power and wealth but also its source is that it is of a divine nature that calls to die and kill for the ongoing survival of the warring tribes / religious ideology. The events of 9/11 didn't make a nation richer, nor did it gain land for the perpetrators, nor did it garner them more power-- in fact, the al-Qaeda lost all three. The sunni vs. shia blood feud has been going on for 1,300 years and is just now, with "better" technology, able to produce causalities that will dwarf all the causalities thus far.
The general populace is less likely to die or kill for the idea that they may get more land or power or wealth-- but if they believe their gods are at stake, they gladly go into the swirling flames of perdition.
In the conflict that is brewing (and brewing larger by the day in the islamist Middle East), it's religious belief that is very clearly at the core. I know the theists are going to jump around and claim that it's the corruption of the religious beliefs and a desire for wealth and power and land, etc., but such finger pointing is a loop then ends up directed at oneself. Ask each of the three groups their perspective, and you will find the majority of each section will assert that in the skirmish:
Moslems believe they righteous and god is on their side
Jews believe they are righteous and god is on their side
Christians believe they are righteous and god is on their side