https://simple.wikipedia.org/wiki/Assassination_of_Theo_van_...
“The murderer Mohammed Bouyeri, an Islamic extremist, killed van Gogh. The reason for murdering Gogh was to oppose the controversial film which criticised Islam: “Submission”. Bouyeri shot and stabbed van Gogh, then cut his throat. Finally the killer attached a 5 letter page to van Gogh's chest with a knife.“
http://news.bbc.co.uk/2/hi/europe/4716909.stm
“Bouyeri had told the court he had acted out of religious conviction. Clutching a copy of the Koran, he said that "the law compels me to chop off the head of anyone who insults Allah and the prophet"”
What was in the letter: it “threatened Somali-born Dutch MP Ayaan Hirsi Ali. She had written the script for Van Gogh's controversial film Submission, which criticised the treatment of women under Islam.”
(Completely unrelated to that event, the killed artist’s great-great uncle was Vincent van Gogh.)