81-year-old man arrested for allegedly stabbing to death his granddaughter 's ex-boyfriend after his granddaughter , amber diaz , and her former partner , kevin rivera , 21 , got into a dispute at their home in brooklyn , new york , in the early hours of sunday .
the recently separated couple , who have a baby daughter , aeralye , together , had just returned to the apartment after celebrating the infant 's first birthday party .
diaz wanted to break up the fight , relatives said .
he has since been charged with manslaughter and criminal possession of a weapon in relation to the killing .