Why didn't Batiatus let Gannicus have his revenge on Tullius?

Tullius, kidnapped, was at the mercy of Batiatus in the sewer...with Gannicus, Spartacus and Oenomaus holding him down.

Gannicus was burning for revenge ever since that party staged fight at the ludus, whereby he was ordered by doctore to lose a duel to the crappy Tullius.

Instead they all stabbed him to death, instead of the ace gladiator being able to humiliate him slowly?