If you want to tear a creature in half, I would suggest a challenge. Each creature makes a strength check, with the defending creature having a +5 or +10 , or even Con +1/2 lvl bonus since ripping a person in half is a near impossible feat. That would make the combat more interactive.
The challenge is there simply because everyone will automatically try to undo the action.
If the defender wins this challenge, they are no longer grabbed, and they are (obviously) no ripped apart.
If the attacker succeeds the creature takes damage equal to half its max HP value. If this turns it into a corpse, the creature is ripped apart, otherwise the attacker can make a second attack next turn, and the defender no longer gets their bonus for defense.
Here is an example:
A troll with +5 STR, tries to rip a halfling(at full HP) with +1 STR, and +2 CON in half.
Troll rolls 20 (15 + 5)
Halfling rolls 18 (15 + 1 + 2)
The halfling now no longer gets a bonus to his save.
Troll rolls 8 (3 + 5)
Halfling rolls 19 (18 + 1)
The halfling is no longer grabbed, but doesn't get a bonus to being ripped until healed, and takes 1d4 bleeding every turn (save ends)... If the troll had won, the halfling would now be ripped in half and incapacitated, taking 2d6 bleed every turn.