Israel wants Assad in power. He's secular and just wants to secure his own power. If the radical Islamites take over, Israel has yet another radicalized country near its border. Israel doesn't like Syria, but they need Assad to stay, but weakened.
It's superficially counter-intuitive, but Israel wants Muslim nations radicalized, thus justifying perpetual US military action there. A stable, and Israeli-independent, Syria doesn't serve that purpose. Their long term objective is total subjugation of the Middle East (They've already accomplished that with the US and Europe), and our military (essentially,
their military by proxy) is to be their primary tool in bringing this about.
