Live as a monster or die as a good man

There were just few people in the cinema this afternoon - a group of high school girls (make a lot of noises but luckily it is before the start of movie), a couple, an middle age woman.
When the light on, the girls and the couple were still sitting on their chairs, but I followed the woman going out. I could not help saying: "It is a really good story." But the reply is out of my expectation, saying: "I sometimes felt bored."
I partly agreed with her because this 2-hours long movie spent almost 20 minutes to explain the truth which Teddy is a created character. I don't know why some said it is an open ending. I think the clues are obvious. For example, in Teddy's memory, his wife was not only burnt to die but she was also bleeding. Moreover, I think the title - Shutter Island is a metaphor, implicating the bad memory of Andrew (Teddy), shooting Nazis in the army and shooting his wife to die, trapped him and stopped him from going back to the normal life. And at the end, I think he wanted to give up this memory by being lobotomized.
From the holistic view, it is a good movie. The ending part, however, is too long to give audiences stronger impression. The music sometimes just distracted audiences from the story.