I am not against the ability to kill people in the game,
but I find it strange if every importent char has a letter with your next goal in his pocket or you can simply talk to his ghost like in D:OS2.
Sometimes players deserve to fail or miss a quest if they mess up a lot or just kill everything.
Killing everyone has the advantage of getting more exp and items, so there should be some kind of downside for it.
Not just sometimes but always. Yeah, people can kill anyone they want, but there absolutely should be a price to be paid, and not just in info or quests lost and no longer being able to advance the story, but also in all the other NPCs in the game world deciding you are an evil SOB who needs to be taken down. So realistically, we should see shopkeepers and other similar NPCs no longer being willing to deal with you, and every single law enforcement person in the world converging on you to take you down. If them's the rules, then sure you should be able to kill anyone.
The one thing your're missing is 'speed of information'.
These day's, it's easy for the news to go worldwide on a murder or manhunt.
In a world like BG3? Or most 'dark ages'-type worlds? It can only travel as fast as a man on a good horse. Or a spell to talk to someone far away.
But in a wild, dangerous world, what man is going to risk his life to ride from town to town saying 'These people are evil'? And what mage will spend his spell slots to tell someone half a world away? Plus they woudl have to KNOW that person to Send a magic message.
So it makes more sense that only people who live near or wittness a murder would get upset. Sending that information to other towns would probably garner 'Eh. They can deal with it themselves' reactions.
Thus, the whole world would not hate you just because of a few evil acts; most would not ever know what you did!