+1
Basically, they need to get the drop on Negan... but then NOT kill him. Something would happen where Negan's presence would save the day, and they'd realize he's a greater asset as a living weapon under their control than if they were to simply kill him.
Of course, starting out, Negan would have to be treated as a prisoner, but in time he could effectively become the wolf living amongst the sheep - and what we saw in Carl's dream would become a reality. And when Negan becomes a step-dad to Judith, he slowly gains her loyalty and indoctrinates her. It would be like what happened in Battlestar Galactica (original) when Baltar was captured and went back to living with humanity. Lots of people would still want Negan dead, so the show's drama would then derive from the idea of is the greater threat from without or within.