Outside of the dream world Charname was always able to control the murder urges but my Charname felt them constantly.
People read the dream sequences differently - have you ever seen the evil versions of the dream sequences in BG1? If you have a good reputation score you let Mulahey pass on to the afterlife peacefully and daddy Bhaal lashes out at you. But if you have low reputation score you plunge a bone dagger into the remains of his soul.
You drown all those iron throne mercenaries in the mine and that night you dream of rivers of blood. And if you are really evil you drowned the slaves as well.
And of course slayer form in bg2 - lose control of it and you will start murdering your friends. So think the "dark urges within you want to be set free" was a pretty consistent theme.
Now you are right that Dark Urge could be artificially created - the Boareskyr bridge water would probably be the best way to do that. That or the body of one of Bhaalspawn killed by charname?
I also occurs to me that the Urge could be Imoen's kid.
You are right that Orin is different - I'm sticking with Bhaal's chosen. Born without his blood but managed to impress him with her dedication to murder.
I don't think Dark Urge in BG3 can lead to that kind or rampage where you kill your friends because Swen said that as the dark urge he did the most evil playthrough possible and he finished with a full party of four, his companions were: Shadowheart, Lae'zel, and Minthara...so I don't think you have to worry too much about playing evil as the dark urge.
There can be problems if you try to bottle up the dark urge as the longer you resist the more you have to start making saving throws...so I don't know what happens if you lose control there. Maybe that leads to terrible rampage? At any rate, don't be nice as the dark urge and you should be good.
