So, I got this idea on the bus home, so I haven't had an time to fine-tune it or playtest it... maybe soon.
Here goes - my attempt to reintroduce an old favourite class from back in the day:
Assassins can wear light armour.
They gain a +3 bonus to Subterfuge.
If they spend three rounds studying a target (taking no other action, except moving at normal speed), they may use a Death Attack with a successful Melee Attack. The Assassin rolls the attack as normal - if they beat the target DC (usually 10+the victim's STR+Phys), the victim is instantly slain. At the GM's discretion, some creatures may be unaffected by such attacks.
At level 5, the Assassin gains the ability to use poisons with no chance of poisoning himself. He may also brew his own poisons.
Using and Brewing Poisons
Any character may poison a weapon - though they may need to make a MIND+Subterfuge roll to avoid inadvertently poisoning themselves (dipping an arrow into a dead creature's poison glands and firing it straight away is easy - pouring it onto a weapon, and storing it for future use? Not so much). Assassins of 5th level or higher are assumed to be capable enough with the use of poisons to not need to make such rolls.
To brew a poison, the Assassin needs to have the appropriate ingredients to hand (GM's discretion as to what is required), and the tools to process them (most likely a mortar and pestle, juicer, etc). The Assassin might be able to make more powerful, esoteric, or magical potions than the ones listed - but, due to the nature of these things, they will need to make their MIND+Subterfuge roll to prevent accidental poisoning. These are rare and powerful poisons for a reason - most who try and make them don't survive, even experts.