Honestly I have no idea how you missed it. They practically beat you over the head with little hints here and there that ego wasn't some nice guy celestial and thought of all other life as less than gnats. Peter was different though because he could've been another celestial and his obsession with propagating himself was also beat into your brain. He greatly preferred and even wanted peters willing help but by no means was he against taking it. The fact that he was so full of himself he didn't get why peter wouldn't bothered by him killing everyone but especially his mother is fully in keeping with his established character. He neither understood or considered killing his mom to be a big deal and was absolutely confident Peter was not a threat at all. There was no reason for ego to withhold info either and plenty of evidence that by bragging about his plans in his mind he was making a convincing case.
Endus is right you missed a pretty horribly obvious part of the movie.