It's the game's way of telling you that you're being silly [img]tongue.gif[/img]
Seriously though, the character is there for a good reason - you're not supposed to kill Aran Linvail at that point in the game or you'd kill the plot.
The thief is equipped with a sword that does about 1000 damage of all types and makes him immune to pretty much anything, so your best bet would be to give it up
Slight spoiler (don't know if this is your first time through...)
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Soon, you'll be given a proposition, and one of the options will enable you to kill Linvail for real. [img]smile.gif[/img]