Why Did They Kill Off Superman?

I’m not a big DC fan, but I went to Batman vs. Superman. I wasn’t impressed.

There was a lot of problems with movie that I could discuss, one of which was the “death” of Superman. I just don’t understand why they did it?

Did they actually think anyone in the audience would believe that Superman had actually died?

It didn’t really add anything to the story. He didn’t need to die for the idea of a Justice League to be born. Was it to show that Superman would give his life in his battle? Just using the Kryptonite spear-thing was enough to show that.  It wasn’t sad, because no one actually thought he was dead. It just didn’t have any effect on the movie at all.

What was the point? Am I missing something? Did the writers actually think they fooled us?