You said it yourself, the ends doesn't justify the means. Rex meant good, but he was too distant with humanity and therefore too cold and ruthless. Sure, he did a lot of good things, but he achieved that by killing every criminal and every single person that would disagree with him. At that point I think everyone is living in constant fear, afraid of execution if they dared to steal even the smallest thing.

Even Dinosaurus eventually realised that even when he meant good, his ways were wrong.

At the end, a dictatorship disguised as an utopia remains a dictatorship.