BW's like to kill BW's (and most other birds). One can take a BW away from other BW's for 2 days and put it back in with the others and one of them is probably going to die. A cage within a cage for about a week so they get accustomed to each other, then open the door to let them mix is one way of doing it. There is nothing wrong with your BW's.......it's just the way they are......they don't like strangers whatsoever.
Do a search on this forum and you'll find loads of posts on this very subject.