In Gun Thunder for .960 by Mirathei, lords can be killed permanently, unfortunately though they wont have sons hunting you down for vengeance (I can see an entire line of family hunting you for killing there father, grandfather, etc.). But be warned in the mod when a lord dies, it will make a grave plot on the world map, don't enter the grave or the game will crash.
As far as I know the repository is still down, Mirathei might have made a rapidshare or something on the Gun Thunder thread though. Regardless of the lords deaths its actually a really good mod, I definitely recommend it.