Ancient mythologies all around the world are full of stories of heroes whose oaths have turned on them. Heck, even Tolkien included the Oath of Fëanor in his mythology, and Fëanor and his sons were no morons.
Boasting or tempting fate seems to be a common disease of males everywhere, and in a premodern society, your word is basically your only property and value. Breaking it is bad, similar to getting thrown into Alcatraz and having all your property confiscated bad.
>> Ancient mythologies all around the world are full of stories of heroes whose oaths have turned on them
First those are dreamed up facts and not the real world, are you aware? Secondly, the "heroes" in that ancient world are a lot closer to "much muscle little brain" typology than such as Euclid and Einstein.
And who the fuck is Fëanor? Some made up fantasy guy? Yeah, that definitely proves your point!
>> Fusiwe located the Pishaanseteri and promised to kill the first man he encountered. That man turned out to be an old friend of his, a young man who had brought Fusiwe's family a lot of game in the hope of marrying some daughter that any of the wives would give birth to. The young man had parted with Fusiwe in grief when the group split. Nonetheless, Fusiwe chose to kill his former friend, because he happened to be the first Pishaanseteri he saw.
Add being a psychopath in addition to severely mentally challenged and you got the perfect combination.