That’s not quite what happened. He explictly named Claudius or his heir as a co-heir to his own throne, along with Boudicca and her sister. The rebellion started when the Romans refused to recognize the power sharing arrangement and (allegedly) raped Boudicca and her daughters.
Wrong bloody thread!
Well quite, the Romans expected him to leave the kingdom to them entirely without any co-heirs involved.
Well, that’s conquest for you. You bring some honey, you bring some vinegar. You reward the peoples who submit willingly, and then you beat the living snot out of those who don’t. Throughout Roman history, you’ll find the first, the latter and everything in between. But it’s not like the Romans were shy about the latter if it came down to it. And it sure helps to have some examples of effective use of vinegar to point to when you’re trying to convince someone to accept your honey.
The Romans were mostly concerned with having at least a nominal legal and moral justification for going to war. That doesn’t mean that they wouldn’t sometimes grab the flimsiest excuse for doing it. And I think we can safely file this one under flimsy as a wet paper bag.
As Gibbon remarked, Rome conquered the world in self defense. 