Islam - their god, Allah, told the prophet, Mohammed, when he had 20 converts, that he should not try to convert people by force... then, when he had gone and gotten himself a following, with 10,000 warriors, Allah changed his mind apparently and decided that force was the way to convert people.
Now, we can compare this to the God of the Bible. God of the Bible has an army of tens of thousands, and tells all but 300 soldiers to go home, because HE wants the glory from the battle.
I guess Allah isn't all that powerful without warriors behind him...
Anyway, those conflicting writings are the reason why Islam can be shown in the media to be a peace-seeking religion while those who follow it can want to convert people through force and kill anyone who doesn't.
Despite history, the Bible never tells Christians to convert people by force. It's actually something that was picked up from Islam, after they came into the area around Spain... when they were kicked back out, people were forced to become Catholic.
It just sort of spiraled from there.
Anyway, today you have people following Allah who believe they need to kill the infidels, so this will only continue.