Nothing, the game isnt counting on the fact that its possible.
It'll just assume he's still alive.
You can kill a certain annoying pirate a million times too,he'll still show up when he's supposed to. If Arkanis Gath shows up to kill you if you managed to screw up the plot with the thieves and you kill him, he'll just keep spawning too.
I do believe there's an improved_continuence-mod that allows f.e. certain people to not show up in ToB if you killed him ... but normally the game doesnt take notice. I guess that the computer agrees with Edwin; Death is usually just a temporay setback ( or something along those lines, I dont have a visual memory like
some people
).