Critic can be used that way, but juxtaposed with fan (and hence the context of reviews), it might not be easy to distinguish between the 'unfavourable opinion' sense and the 'professional judge of art' sense when you hold both terms on equal footing with the phrase "fan and critic".
1 A person who expresses an unfavourable opinion of something.
‘critics of the new legislation say it is too broad’
2 A person who judges the merits of literary or artistic works, especially one who does so professionally.
‘a theatre critic’
You can weight the interpretation more heavily towards the 'unfavourable opinion' sense by noting the process of change, juxtaposed with the word 'sad'. For example:
- Its sad to see how fast fans become critics.