"So by your theory, if there was no concept of a god, people wouldn't want to kill each other as much"
The animal in all of us, particularly the "pack" animal, ensures there will always be killing of those we see as different or a threat. Organised religion often seems to emphasise or even organise this trait.
"Do you think Islamic fundamentalist terrorists are killing in the name of Allah and using more radical passages from the Koran as the reason, or are these things just convenient excuses?"
When religion is taken this seriously, the subject individuals are told, and believe, that not only are they acting for their god but also that they will then enjoy an afterlife, all the better for having died in "battle". Suicide bombers for instance don't believe that they are truly going to die, they have swallowed the "you'll get at least a hundred virgins in heaven" type line

So the answer is probably both, and this type of activity will always be present whilst segments of the human race allow individual ignorance to become organised religion.
There never were any fairies at the bottom of the garden, however attractive the notion.................