One of my parakeets killed the other but they got along so well?

my boyfriend baught me two parakeets ive hade them for about two years more or less. they always used to fight a little bit especially right when the sun came up. but they used to groom eachother and play and got along. This morning i woke up and looked into theircage and the girl bird was laying at the bottom and i was so upset. i dont think she died of natural causes whn my boyfrien got her out and i was placing her into the box we looked at her face and all the feathers on one side where plucked away and her eye was peacked out. so im uming he killed her or she died and he went to the bottom and removed the feathers and pecked at her eye. i know it isnt unusual for birds to kill one another but why all of the sudden. i didnt give them as much attention as i should of they bit so i baught them a hudge cage so they would have alot of room. and i played music for them and they loved it. should i get anothr bird or will he kill that one too?