I wouldn't say stupid just over confidant. At this point he thinks he has all the groups in the area scared of him and has them in submission. I doubt the Hilltop or the Kingdom would have taken those explosives. They are too scared to piss of the Saviors. He thinks Rick is like that too. I am sure the other groups feel the same way. No one wants to die or get one of their group killed. If anything Rick was kind of stupid because for all we know there could have been a look out hiding somewhere in the trees or elsewhere. That whole scene was kind of dumb.

Negan must be really stupid to have them somewhere where they could take them.