I don't actually think he was being clever, because I think he's too impulsively cruel and self-centered for that. I think he was wiley enough not to tell his mother but not that he'd really thought through any implications. But it also seems perfectly plausible that Littlefinger, knowing how Joff behaves, whispered a word in his ear that cemented Joff in his desire to be the nasty brat he's been from Day 1.