Why did Leslie die offstage?
I think that Leslie died offstage as Bridge to Terabithia is a children movie, it might not be appropriate to show a scene of how Leslie died. It might also be that when Leslie died, nobody even witnessed it so it is not possible to describe and show how Leslie died. Therefore, that is the reason why Leslie died offstage.

What does the Dark Lord represent for (i)Leslie?
To Leslie, I do not think that the Dark Lord represents anything in particular to Leslie. In Bridge to Terabithia, we can see that Leslie is a child with a vivid and wild imagination. To her, the Dark Lord is just one of her imaginations and to me she is not troubled by anyone or anything.

(ii)Jess?
To Jess, the Dark Lord would most probably represent his father. Both in the movie and novel we can see that the relationship between Jess and his father is not as good. Jess always tries to do his best in everything to impress his father as he has this mindset where his father thinks that Jess is not able to achieve anything and is not proud of anything he does. Instead, he would only spot Jess' weaknesses and the things he did not do right. The father does not take the extra effort to get to know his own son even better and built a stronger relationship with him. To Jess, his father is like a stranger who lives with him. We can tell by the way they talk to each other, there is not a sense of comfort between them and they are never open to each other when they are supposed to, as father and son.

(iv)me?
To me, I think that the Dark Lord represents Janice Avery. Janice Avery is known to have went into the woods where Terabithia is. Janice Avery is also known to be a big bully in school and almost everyone is afraid and dislike her. In the movie, she is seen as a mean person and could have represented the Dark Lord. She is mean in many ways. For example, she guards the girl's washroom and expects them to pay up in order to enter which is so unacceptable, she bullies other students like throwing bread at them and steals small girl's snacks. Therefore to me, the Dark Lord represents Janice Avery, the big, mean bully.

Do I prefer the movie or novel?
For me, I definitely prefer the movie to the novel. I think I enjoyed the movie alot. For me, I got to understand the story of Bridge to Terabithia even better after watching the movie itself. I guess, if I had not watched the movie, I would not have known that the Dark Lord even existed in the whole story. With that being proven, I guess that watching the movie gives me a better understanding of the whole story. I prefer the movie also because as I watched the movie, I could feel the sense of sadness, happiness and all kinds of emotions, not like when I read the novel. Actions definitely speak louder than words. I like the movie better as it gives me a clear concept on the relationships between the characters. The movie allows me to witness how the bridge was and all the other elements in the movie. Especially the Dark Lord. Since this is a movie full of imagination, to understand and to visualize it better, it is definitely better to watch the movie itself. So my final conclusion is, I prefer the movie more than the novel.