If you can't say something nice, don't say anything at all???

Is it fair to tell someone that their answer to a question posted here is wrong or stupid? Is it your place to decide how everyone else should answer? There are so many different ways to view a lot of questions, but when someone answered in a way another didn't like earlier today, they were called 'negative' and insulted. On a scale of 1 to 10, how "ok" is it to diss another person for their opinion?