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.
