Israel frequently bombs Assad's positions.
Israel has been a bad actor in this context.
And that's because Israel wants Assad's war machine greatly reduced in power. But you're making the mistake in thinking that just because that's what they want, they also want him overthrown. Israel knows that "moderate rebels" replacing Assad in such a case is a pipe dream. As I said, in the power vacuum created in the event of the regime's collapse would quickly be filled by Islamic state loons, which certainly wouldn't be in Israel's interest. ...or in anyone else's except radical Sunnis. Caliphate types controlling Syria's chemical weapons stockpile isn't a sane option.
Therefore, they seek to keep Assad in power while at the same time seriously degrade his military. At this point (years into the fighting), it's close to mission accomplished -- ISIS and Co. isn't going to overthrow Assad, but the Syrian military is a shadow of what it was when the war started.
Ex-IDF chief Dan Halutz publicly expressed the truth 3 years ago.
I'm sure he was told never to do it again.