Sometimes they are aggresive when you aren't around. I have been surprised by fish aggression many times and sometimes it's the little shy ones that are just vicious. Remember, they have speed and agility on their side if they decide to attack the giant in the tank.
Anyway, that bump makes me wonder too. My first thoughts are parasitic, tumor, or abcess. If it were an abcess I'd expect it was from a fight injury, but her tail being so evenly "nipped" makes me think there's more going on..
Test your water (I'd test everything), make sure nothing's off. If that's all where it should be, only an autopsy would really tell you anything more.
Sorry for your loss, she was a beaut.
*edit I also freeze fish if I have to euthanize, if it isn't -10 outside I have a chest freezer that's well below freezing and they go in for a whole day before I throw them.