About executing lords feature, I've thought of several penalties for making it more balanced:
Big relation loss with the faction: you could have a -40 or so relation penaltie for executing a lord, also when you execute a lord it means you have already negative relation with that faction because of war.
Relation penalty with any lord that didn't hate the one you executed: you would loose 10 relation with any lord that had a relation over -40 (or other really low ammount), this could also chage for each lord depending on the lord's character.
Even greater relation loss with every lord belonging to the same faction: something like losing 30 relation with them, it could also vary for each lord depending on their nature.
Absolute hate from their relatives: maybe lossing 100 relation? Or directly having -100 relation with them?
Huge loss of honor: In Warband this is almost useless but maybe in Bannerlord it will have some more use.
You could also lose a lot of influence, who would follow a crazy dude that just kills the lords he captures?
Maybe moral loss? Your troops wouldn'd like that, or maybe they are just as sadistic as you?
Maybe losing relation with some settlements?
All of this wold make it possible to execute one or two, but if you go in a killing spree even your own vassals will end up hating you.
Also some pros:
Renown increase, well, it's an effective way to make more people know you, isn't it?
He won't be able to make impossible escapes anymore.
Also, hi everyone!