Is Israel on the side of the Rebels or are they simply trying to keep Syria from becoming proliferated with arms on either side?
“...Is Israel on the side of the Rebels ...”
I believe not. They most likely can predict the outcome. But either way, they do not want advanced weapons falling in the hands of wrong group.
I think the latter half of your questions is a good observation.
Up to this point Syria and Israel at least appeared at “peace”.
(note appeared)