The Mero and crew never really had problems with the former "ones". The Mero has "survived" them all. So he's already certain that he will survive this "one". Even though the keymaker is let out by Persephone, they will need Neo. Thus, assuming that the Mero will kill Neo in that scene, there is no more need for the keymaker (maybe that's why the Mero didnt get mad at Persephone. He's certain it would be game over for Neo). The twins get hold of the keymaker again, lock him up, and etc. But he never knew of Neo's absolute strength against his crew, so everything just turned upside down for him.
"Mark my words, boy, and mark them well. I have survived your predecessors, and I will survive you! "