Eitri seemed less like being dumb and more like Thanos acting somewhat out of character. It's established several times throughout the film that he lets half of his victims go, and he genuinely seems to have no malice or enjoyment in killing people. And he's true to his word, as we see during the fight on Titan, where he spares the lives of the Avengers and Guardians after Doctor Strange gives up the Time Stone.
So the fact that he made a bargain with Eitri and then slaughtered his race anyway seemed kind of weird. I can only assume this is because he didn't want them to be able to forge a weapon that could potentially ruin his plans.