I have always felt that Catherine sort of used Paul as heir to get the throne. He was really the heir, because he officially at least, had Romanov blood, and was the recognized legitimate son of Peter III. So, when Peter was dead he was the next heir. Of course, during his minority someone had to rule. When Catherine took the throne, she may have made it look like she was only ruling for his minority, and I am sure many people thought that. But, she ruled long past that, and he never got his fair chance at the age he should have. She never recognized him as ruler, true, during the years of his minority which was usually done even when the person was not actually ruling, due to their age. This might have been the first tip off. But, of course, by the time he was of age, she was there to stay. He may have resented that, as he got older, and perhaps his links to his father were important to him in part because it reinforced his claim to the throne, although he would have never dared voice that in her lifetime.