Yes, perhaps there are a lot of people who agree with you. But that does not, however, mean that the majority of people who saw the movie disliked it. The implication, whether you meant it or not, was that of all the people on planet Earth who saw the movie, most disliked it. There is simply no way for you to know that. Your initial post was speaking on others behalf, again, whether you meant to or not, you did. Saying "we" instead of "I" is what made it come across that way.
And, you're right, you don't have to prove anything to anyone, or justify your opinion on the movie to us. But, by the same token, don't expect to be able to make wild, generalized claims, without someone calling you out on it, and asking you what you're basing it on. Usually, when people generalize and make big claims like that (assuming everyone agrees with them), whenever someone asks them what their source is, they take the attitude of "I don't have to prove anything to you". That makes it clear that either A) you don't have anything objective to base your claim on, or B) you know deep down inside that you can't back it up, and are instead just trying to justify and validate your opinion, which you don't have to do anyway.