Don't you mean you
do? The idea that the Son'a would let the Ba'ku live is so out of the left field that I can't fathom how you could get it from watching this movie.
The events progressed from lie to lie, until at the very end the truth was revealed. And the truth was that this was all a blood feud.
Picard: "You brought the Federation into the middle of a blood feud, Admiral. The children have returned to expel their elders, just as they were once expelled. Except that Ru'afo's need for revenge has now escalated to parricide."
"Escalated" is Picard's estimate. But consider Rua'fo's earlier words:
"Just don't forget what they did to us. ...We'll have them rounded up in a day or two. We needn't bother with the Federation holoship any more. Just get the holding cells ready."
Obviously, the holoship was never going to reach its destination, as the Ba'ku would then get a chance to reveal the truth to the Federation. The Son'a would know that the Ba'ku were no primitives: they could not be fooled with holograms into believing they were still on Ba'ku. And the Son'a would know the Federation would not like the truth, as well evidenced by their many threats regarding that, Dougherty's counterthreats, and the action taken against Riker's ship.
The case appears airtight. The Son'a were out to kill, and there was no way they could not
kill and still expect to survive.