Even with the penalties to all lords, it doesn't matter, executing everyone in sight is the best way to reduce (it's still a massive, massive grind given the insane amount of extra young lords/ladies that will become adult as years go by) the tedium if you aim to conquer the whole map.
It doesn't matter both because it trivialize the difficulty in achieving it and because having such severe penalties with everyone doesn't have any real impact on the gameplay at all. Plus if you need at some point to raise the relationships all you need to do is fight the lord/lady and release them instead of making them your prisoners. Takes maybe 4 times or so to go back from -100 to 0.
If anything there should be a system that should give you an acceptable reason for executing someone, like having raided your fiefs multiple times by looting/killing the population in simplistic terms. If there was anything else to do in the game during peace times there could have been more reasons (such as adultery for example).
Otherwise being able to execute freely like this, feels like having an easy mode cheat code since once you get to a certain stage of the game, I never lose a battle anyway (by fighting only the battles I know I'll win until I become so powerful that nothing is a match anymore).
Edit: I had missed kweassa's post, basically what he said. That's the only way executions would make sense.
The only other one that is currently "justified" by default is the execution of rebels, and the AI does execute them indeed, although I wonder why we still get (as a player) penalties for executing them when the AI doesn't. Probably wouldn't matter either way, but in this case the player shouldn't lose any at least for rebels.
Edit 2: I am adding something that I have noticed while playing myself: if you have your own faction and you execute lords from a different faction, you'll still lose relations with your own lords, but only if they are neutral or honorable. If you have in your ranks a devious lord, they don't seem to care whether you execute the whole calradia.
Not that it seems to matter anyway, it doesn't seem like lords in my own faction that have a -100 relationship with me can actually defect, or at least it hasn't happen in my game where I went on from a couple of fiefs to owning the whole calradia.
It never seems worth it to me, on any character playing any possible role.
