No - you do not. I did not execute a single enemy lord during my current ggameplay (more of an RPG fun than conquering all mission for me). I even reverted to saved files few times when enemy lords died while fighting me... And I had no problem beating up opponents during war (though because of my kingdom military strenght they just do not declare war on me and when I do - they agree to peace fast). Fast and massive unit spam is the only strnght AI got - can't take that away from them until their gamplay improves!
Guys - just read this forum - plenty of usefull things can be find thanks to other players. I did - really made this game a lot easier for me. There are informations about game mechanics, best tips and tricks - all you need...
But since you probably ain't gonna do that I'll make things easier for ya:
Hire 2 or 3 mercenary clans - make 1 massive army and steam-roll.
Need to build up your own ranks? Wait for 2 big armies to clash, stand next to them, declare war upon and wipe-out winner - you get 200+ units instantly... When your mercenries parties get low on soldiers - let them go and hire new ones with full ranks.
Also with new rebelion mechanics - you can get towns without messing with larger nations.