This, and the attitudes and behavior are why faction locks should be implemented (again).
The game used to have some form of faction locking, but it was removed so people could play with their buddies. Which, hey, I get that 100%. That's why we have an ES MMO.
However. Now, we've got BGs. No faction locks on that. You can pvp with your buddies on your fave characters again.
Cyrodiil, on the other hand, gets the short end of the stick after testing this "no faction lock" style MMO. It just leads to a lack of faction loyalty, "griefing" such as OP and MANY others have witnessed, and demoralizing game experiences.