He would like to explain that the murder had been the only thing to stop GLaDOS from killing them and that those cores hadn't had a life and couldn't function without being plugged into GLaDOS anyway... and it had really hurt to kill them but he understood what he was doing and had to do it anyway.

But Chelldon already played with being stupid and damaged so he didn't ruin the illusion that Wheatley clearly loved and wanted. The core just wanted him to be a pet.

And Chelldon couldn't deny that he sort of loved being that because he'd never ever had a friend. Except Walter.

So instead of apologising or doing anything that could imply intelligence, he just wrapped his arms around Wheatley and jumped till the core was forced to jump with him.