"I have weighed my options. My desire to kill her outweigh my desire to know more about myself and this world".
Sango rationally thought of what was most important to him at the moment before he made the decision.
Zita's death at his hand might end up leading to the guild leader amassing his forces and making their way to the forest to hunt for Sango.
While he is not scared of an outcome like that, he still has to apply a certain amount of wisdom in dealing with such situations.
The number behind the guild leader's forces, their combat power all together, their resilience when it comes to hunting down a fugitive.
Sango took all of that into his thoughts before making his decision.
He based it on the possibility of the guild leader being adamant when it comes to chasing after a fugitive.