It depends on how you are watching the movies online for free. There are some movies that are freely available to watch online. It would not be illegal, for example, to watch a movie that is being promoted as free to watch. There are some websites that offer free movies that are uploaded illegally. It would be against the law the watch a movie online that was uploaded illegally, however the bigger crime is the uploading of the illegal movie in the first place.
Some sights are obviously uploading content illegally For example, if the movie was obviously filmed in a movie theater, that would be illegal.