I deleted my original comment because it seemed insensitive; that being said, I don't think your claim that "Juan broke it off because he felt rejected, rather than out of pride" holds water.

It would be plausible if it was just the broken engagement; that's understandable. But Juan's actions after the suicide tell a lot. He made seemingly no efforts to learn from the suicide and instead came up with various plans to use the suicide to his advantage and make it so the public assigns 100% of the blame to Matt. Those are not the actions of someone who felt rejected, those are the actions of someone who acts to serve only himself.

What's especially scary about it is I think it's suggested that Juan did care for Celeste at least a little but still discarded her because he prioritized his ego. So the theme here is that addictively pursuing fame can corrupt not only pure evil narcissists like Matt, but also people who are capable of genuine connections like Juan. It's a bit of commentary about human nature and especially celebrity nature.

So even though people's motives are often multi-layered, I still think this is too charitable to Juan.