“Donors in Saudi Arabia constitute the most significant source of funding to Sunni terrorist groups worldwide,”
“More needs to be done since Saudi Arabia remains a critical financial support base for al-Qaeda, the Taliban, LeT, and other terrorist groups, including Hamas, which probably raise millions of dollars annually from Saudi sources.”
It's fertile ground for raising money to support terrorism not because a few bad apples support but because it enjoys broad support. Although data is somewhat scarce a survey in 2003 found that 53% supported Bin Laden for example.
https://icct.nl/app/uploads/2017/02/ICCT-Schmid-Muslim-Opini... Page 18
Which is probably why for example US made weapons have found their way via Saudi Arabia to al-Qaeda and ISIS linked groups.
https://www.aljazeera.com/news/2019/2/5/saudi-arabia-uae-gav...
Basically the bad apples are in fact somewhere between a majority and a near majority and they control the country.