“Israel is backing ISIS?”
Israel is engaged in realpolitik with regards to Syria.
Fundamentally they view Assad as a greater long term threat than ISIS and Al-Qaeda. Assad is a Shiite and on friendly terms with the Shia in Lebanon (Hezbollah) and Iran (Republican Guards etc.). Israel considers Iran their number 1 threat at present. Therefore, Israel sees Assad as a greater threat than ISIS. That is why they bomb Assad and treat injured Free Syrian Army (Sunni Jihadis) forces in their hospitals.
http://www.timesofisrael.com/the-curious-case-of-israels-syrian-patients/
But supporting ISIS and AQ for geosyrategic reasons, especially with the goal intended, is gravely immoral IMO. There a fundamental principle in Christian moral theology that says that it is never permissible to do evil so that good might come. If one were to call a geostrategic gain good when is has the double effect of slaughtering Christians and other non-Sunnites.
I thought Assad was an Alawite Muslim.