I don't think players should be able to kill their own guards without any penalties. It just doesn't make sense. The guards are there to protect the cities and if you attempt to kill a guard it would seem logical that an APB (all points bulliten) would be issued and all the other guards would begin to hunt for the attacker.
I think it's time to consider implementing faction into the game.
For example: if a clanner kills a clan guard, you would create bad faction that would cause all the clan guards to attack you on site.
To remove the bad faction you would have to kill, say, 30 omni guards of equal level to erase the previous bad faction. Then you would not get attacked by clan guards.
This would not prevent guard killing but it would be an appropriate penalty with a way to erase the faction.