I don't quite get how his plan would lead to the prince killing diana. Like, it not only depended on diana's magic going out of control, but it also required the prince to deem her enough of a threat to people (that likely left the area by themselves) that he needed to actually permanently put her down.
I might be getting it wrong, since to get a better idea will need to reread Diana x Rubeus interaction from a previous chapter, but from what I understood.
The point is to push Diana towards becoming a villain of the story by leading her down the path of committing a number of mistakes and becoming a criminal while simultaneously persuading the Prince that the only available punishment worthy for a criminal of such magnitude is death. Don't forget, Diana is a powerhouse in this story.
That, or perhaps it was to create an opportunity where Diana's power goes out of control, which if I recall right have already happened before, and having Prince interfere by killing her.