I suspect it's the latter, a "loyalty cascade" (as Cryp18Struct said). The dwarf transformed was a friend of some of the guards, and those friends then attacked the killer.
Try to isolate those bit by were creatures when the next full moon occurs (bricked in or drawbridge enclosed. Doors are not enough). Those who are not transformed should be safe and can be let out. Those that do transform can be dealt with in several ways:
- I think they can be convicted via the justice system, but I have no first hand experience.
- Atom smash them.
- Use them as expendable cavern explorers/shock troops. If they survive, any damages suffered will regenerate fully at the next transformation.
- Treat them as unfortunate victims and lock them up over every full moon, but let them out otherwise. Will probably be a lot of work that will fail at times. It should be possible to use pressure plates to deny them access to areas when they transform, since the were form is probably much heavier than a dwarf. Haven't tried that, though.
- Put them to work in isolation. You could device a mine cart or chute based system to get stuff in and out. Also note that weres of the same type don't fight each other, and so could be placed together and act as company (don't think weres can generate offspring, though).
- This might be the chance you've been waiting for: You can transform the whole fortress (some people do that)!