If you want to PvP with pets, you should go in the UO options and change the default colors for flagged people so you can make the difference between someone who is grey and someoen who is flagged to you.
What happens is that by default, both appears grey. The tamer and the pet have different flags. This means that you may be flagged to the grey but not your pet. By commanding your pet [non flagged] to attack a blue flagged to you, you order your pet to make a criminal act.
This feature is a lot abused to pas murder counts to tamers. It can be abused too against people using followers like demons, energy vortices (EV targets a grey guy, but hits the guy when he is blue a few seconds later), etc..