These people have started a war, they brutally murder everyone who disagrees and proudly display their heads.
They rape children and sell women like cattle.
What "Noble ideas" about Islam were you going for then?
The big appeal to ISIS is that it's an Islamic State. That might sound redundant, but when you consider groups like Al-Qaeda and their call for suicide-bombing jihad with 72 virgins in paradise as reward, the notion of building your own country for your culture and religion is a much more positive and appealing message.
It's not like a lot of western Muslims go into it knowing all the facts about how ISIS brutalizes, rapes and murders scores of innocent people, women and children included. And even those who do know that ISIS is violent may see it as justified given who they're up against -- it's not like Assad is some patron saint.
There's a reason ISIS' recruitment pull is strong. They operate like any other cult: find people marginalized in their current societies, appeal to their base desires, capitalize on their lack of awareness, and draw them in with promises of a better life and future for them and their families.