Who are you?
One of the most significant factors in this scenario is who we are, yet it is not presented. It appears to be the case that we are a god of sorts, controlling the situations as if the people were our playthings, however then why is Pro applying human emotions and morality to the scenario?
I hypothesise that in Pro's scenario, we are a demigod in fact and could become the leader of both tribes if we so wished. Absolutely nothing rules this out. Consider this Kritik a short and simple one.
Isn't there chaos regardless?
Something the 'suggester' leaves out or fails to mention is that there is chaos regardless and unpredictability in the tribes no matter what. In fact Pro makes this very clear by the fact that he advocates integrating the leader of the tribe you just slaughtered entirely... Yes, he thinks you should kill women, children and babies (as well as innocent men) and to then say 'hey leader, give your expertise and assistance over to us'.
That is chaos if I ever saw it, the kind that will appear peaceful for years as this leader bides his/her time to unleash all hell broken loose on the tribe that annihilated his own.
On top of this, it would blatantly be clear that the tribes are at war, right? These tribes are very passionately hostile with one-another but if suddenly you wiped out an entire tribe in the blink of an eye (bar the leader), the other tribe would begin to panic, think god is about to curse them, think their leader was behind it and not forgive him/her for killing the younger members of the other tribe.
All kinds of chaos, mistrust, heated emotions, rebellion/coup can occur as a result of this. Even the other tribe leader's closest allies within the tribe may coup him, for they'd assume he/her unilaterally arranged the genocide of the other tribe without consulting them first.
I'm not saying chaos is inherently bad, especially as it seems we're just a bystanding demigod enjoying the show, I'm merely rendering the chaos point moot.
Who is responsible for the situation?
There has arisen a scenario where both tribes want to kill each other very badly. Who do you blame other than the leaders?
Why is it 'suggested' that they are the only ones to lead the tribe well, when in actual fact they have proven to do so quite terribly (at least one of them has).
We must take out the two men/women responsible and allow the others to either come up with a new leader, come up with an anarchic system that works or to seek reconciliation with the other tribe and fuse (for all we know, it's literally the leaders who are forcing the rivalry to continue).