I'm really lucky in that I live in a reasonable part of London (well you can't even call Richmond London as it's Surrey, but still on the tube), also I tend not to travel around on it too late. But even so there's been one incident (October 2003) where I was pretty frightened in a carriage on a tube not even that late at night.
The problem with fights on the tube is that you are in such an enclosed space that other people can get caught up in stuff by accident. Tempers do flare and it's amazing there isn't more tube rage. The other problem is that with this retitcence and keeping yourself to yourself on the Tube people ignore violence. I'm not saying if this is right or wrong - I'm consider myself a scaredy cat and normally tend to move away if I have a sense of trouble erupting (not sure why I didn't in the October incident).
Anyway, must shoot off to work myself now, and for what it's worth, I would like to offer my condolences to the family of the man who was murdered.