If you like backstabbing, you should IMHO choose the kensai/thief instead of assassin. It can backstab better (at least in the 6/7 part of the game), and it has much better THACO and can be grandmaster in a weapon + the kensai damage bonus and kai. You can use the kensai/thief as a fighter too.
The drawback is that you don't get thief-skills in the beginning.
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Hmmmmmm...Yes, that must be it