Mark almost falling from tree

For those that did not read the novelization its a good chance Henry set it up to kill mark. In the book something interesting was mentioned. When mark gets back down on the ground he sees the branch that broke under him and it looked as if someone started cutting part of it with a saw. Very interesting although mark does not fret about this. I believe there was only one reason he did not let go of marks hand. His sister had wondered to the bottom in the movie and was staring up. She would have seen if he deliberately let go. Henry might have wanted to kill mark just for the annoyance of having to share his room and things for two to three weeks. Another idea is he surely got a thrill of it and he knew this would look like a accident just like his little brother.