Ronald, that's a valid point, and has a valid counter: why should Israel allow Hezbollah access to more weapons if they can prevent it? Syria is aiding and abetting the arming of Hezbollah, and for that matter, Hezbollah is fighting shoulder to shoulder in Western Syria with Assad's forces.
It's not as black and white as you'd like it to be.
Civil wars are messy enough, and are even messier when outsiders get involved.
See Spanish Civil War of 1036-1939 for a stellar example.