This is a subject-verb agreement / pronoun function question

This is a 2-part Subject-Verb Agreement question;

I am wondering if the word 'that' is functioning as a relative pronoun, or as a subordinating conjunction in the following sentence; 

"The fact that historians have caused so many atrocities have caused some historians to adopt a cynical perspective on human nature."   

I ran this sentence in the grammar checker and it came up as error-free, but I am thinking that there is a subject-verb error in that the sentence should state; "...atrocities has caused some historians...."   

Any feedback on these two issues is appreciated.     Angus