If you dont like a movie, you can always walk out of the theater and get your money back. As long as you do so before the end of the film, they should refund you your ticket price.
Other than that, if you see the whole movie, you should be paying for it. Last time i checked that's how our system works. You dont get to pick and choose what you pay for. If you take something, you pay for it. You dont get to try it out, and decide to go pay for it later only if you like it.
I can just imagne you walking up to a counter at a retail store, "yeah, i stole this pair of shoes a while back, just to see if i liked them, and it turns out i do. so i'm gonna pay for them now."