I'd be tempted to use the victim's Fort save as well, but run it as an opposed check:
I usually tend to go the other way. Adding another Dice Roll rarely adds anything that an increase to the DC can't do just as well, but it slows down the game ...
HOWEVER
In this case, if a PC were being torn in half (and presumably killed) they should be given a chance to make a saving throw. While this might not greatly change the odds of the character surviving, it means that his/her survival is dependant on the player's roll, and not the GM's. This will make a big difference to how the player feels if their character does end up dying.
Ripping someone in half:
You must deplete HP to the point where they are dead. Also, no rules on this, but if we arbitrarily say that dead is an intact corpse, lets say dismemberment happens at twice that damage amount?
An average human (commoner) at level one will have @6HP and a CON of 10.
To kill them, 16 points of lethal damage is required.
So, to 'rip them in half', without having rend, natural attacks etc, you would need to inflict 32 points of damage (lethal damage).
So, adding the original 6HP to this number (you have to do the full HP in non-lethal before it rolls over and becomes lethal), we have 38 points of damage.
Unarmed attacks deal 1D3+STR bonus, so, we need a minimum +37 to guarantee a kill in one hit.
STR 84.
That is without resorting to saving throws, grapples, rends etc. It is a guaranteed "Twice the HP needed" kill. On a level one commoner.
Under no circumstances should you ever allow an 'automatic kill if you have STR X' ability. HP are a basic mechanic, and should not be ignored.