Conceptually, poison is a killing attack. The difficulty is getting the poison into the target, but once it is there it is devastating. There should probably be lots of negatives on the attack side, but then lots of damage on the crit charts.
Using a poison tipped arrow... The liquid may come off the arrow in flight, when striking the target (even plain clothing may wipe some away), when the puncture strike is converted into a crush-like damage but never pierces/touches skin, etc.
Now an ingested poison is far worse because there are no reductions. Once someone swallows it the battle becomes internal where the individual's resistance and fortitude are up against the poison's strength.
There are no other attacks that I am aware of that if they strike successfully have such a high chance of death - and it is reflected in the crit charts.