Either way, the goalposts seem to be shifting as objections are raised. There's no motive; the motive was cruelty; the motive is complex.

You think Jaime is absolutely wrong on the premise that Joffrey's motive was some sort of logical XOR function, where only one or the other could be possible. I said it's complex. It can be both. But you can't deny that cruelty is a motive, since we're talking about sending a man to kill a child in a coma. Cruelty defines Joffrey almost from the beginning of the story. Cersei and Jaime just add a possible deeper motive in their scene.

I mean, if we applied your logic, nothing would be possible because someone got one fact wrong. Yet things happen while characters get some facts wrong. So. I don't know what to tell you, man. Sorry if you don't like it that Joffrey did it? But it's a fact.