Actually come to think about it, the lead writer's playthrough had Shadowheart solo and he said she was more powerful because she had killed her teammates. I think you absolutely will be able to kill some if not all the origin characters as early as act 1 for their tadpoles. Wild.
The evil route of Mask of the Betrayer had a similar mechanic when the cumulative effect of your desire for power lead to more and more heinous acts until almost all your companions turned on you and you were despised by all
I wouldn't be surprised if there's an ending where you end up an abomination to all, illithid and non alike, either with or without any semblance of who you were before.
