I have noticed this. Messi and Ronaldo are both great players, and they have their own strengths and weaknesses. In order to become a Messi fan, you don't need to hate Ronaldo (and vice versa). And I don't think that there is any personal hatred between these two players. The fans need to understand that. All that said, IMO Messi fans who hate Ronaldo are more common than the other way around. And what would be the reason for this? Nothing comes to my mind right now.
I also don't understand why there are Messi fans hating Ronaldo when Ronaldo is a little older than Messi in terms of his football career. It means that both players have developed individually without copying each other so there is nothing really worth hating them even though everyone can see and make comparisons about what they have achieved in their football careers. But I am really impressed with Ronaldo's success in the Champions League to the point where he got a special award after being able to perform well in 183 appearances.