F6 Male Savannah 3 yrs. old suddenly aggressive

I have a problem with my housemates Savannah cats.  She has 3 males ranging with the F3 being 4yrs. old and  2 F6's being 3 yrs. old.  The F3 is a moderate tailbiter, but the F6's are far worse.  One named Pistol attacked one of my cats, injuring him causing his death a couple of weeks later.  I thought I got to my cat in time, but evidently not.  The other F6 was suppose to be mine and was sweet, but suddenly turned aggressive to the others.  Meanwhile the F3 is not violent as the F6's.  I love my  Sav., I don't know what to do with him.  Sweet one day and nasty the next.  

If anyone has suggestions I would appreciate them.  I do not have faith in the vets here, because I believe 2 of my cats late last year were killed by the vets in the area,

Thank you

I am so sorry you are having such terrible problems.  Are the cats neutered? 

You may need to keep the cats separated if there is such sudden aggression.  Also a vet visit might be in order. 

Neutering can be an issue here. Cat aggression to the point of killing another cat is very rare, so I do think the aggressive cat needs to be seen by a vet to rule out any medical issues. I would also talk to the breeder about this. I don't think any cat sold as a pet should be this aggressive.