So why did his kill?
One reason could be that he had too many males in too small a tank. Male angels are much more aggressive than females. They are territorial by nature, choosing a specific part of the tank - or even a particular plant - to guard. Adult angels should have a minimum 10 gallons per fish.
Female angelfish tend to be smaller and they are far less territorial - unless they are actively laying eggs and trying to protect them with their mate.
Another reason his angels could have killed would be to protect their young. I have four angelfish in a 60-gallon tank where I used to have six, but when the koi angels became too aggressive during spawning, I removed them to their own honeymoon tank. When they were in the community tank with the others, they aggressively pushed everybody to the far corner of the tank while they claimed the rest of it. But their aggressiveness never even led to fin nipping.
If you're looking to purchase freshwater angelfish and you'd like to keep several in the same tank, look for the smaller angels. If they are all from the same clutch, the smallest ones tend to be the females.
Observe their behavior. If you see one going after the others, that aggressiveness will only intensify as the angel gets older and larger.
If you want only two angels and you would like for them to breed, select two of the same coloring and choose one large one and one petite. That's no guarantee that one is a female, however, so if the larger one begins to bully the smaller one, they should be separated. The only way to know for certain that you have one male and one female is during mating.
