The West faces a choice in Syria. If it wants to destroy the Islamic State (IS) as its main focus, then it should agree to tolerate Pres. Assad for the time being, until significant gains are made against IS.
If the West, and mainly the US, continue to divide their efforts both against IS and against the Assad regime, then IS will to some extent be let off the hook. Russia is also divided in its approach, attacking all enemies of Assad, including the so-called democratic or non-Islamist opposition, consequently they too are not focused only on destroying IS. After the traumatic attacks in Paris, Pres. Hollande of France, is now understandably trying to build an international coalition to focus only on destroying IS. But, he has not had much success, because Pres. Obama will not agree to allow Assad to be immune from aerial attacks and Pres. Putin will not agree to allow the non-Islamist opposition to be free from their attacks. In Russia, Putin asked Hollande to supply him with a map of the “non-terrorist” opposition to Assad so that he could avoid attacking them. Whether or not he will do so remains to be seen, but it could be a step in the right direction.
From the Israeli point of view, Assad is a better choice in Syria than the IS. Assad has been a pragmatic enemy of Israel for a long time. While he was in power, although he is a ruthless and despicable dictator, he nevertheless avoided direct conflict with Israel. Presumably he was afraid of the destruction that Israel would cause as it did previously, taking out much of Syria’s infrastructure and destroying his air force (the last war they fought Israel downed 85 Syrian plane to 1 for Syria). He preferred to support Hizbollah, a Shia ally in Lebanon, in attacking Israel, so that Lebanon got hit instead of him. Israel is great at fighting a State enemy as opposed to a terrorist enemy. So it is far better for Israel to have the pragmatic non-Islamist Syrian dictator on its northern border than to have the irrational terrorist IS enemy. It would be ironic that Israel would be supporting Assad to be left alone by the Western forces in preference to their hitting IS.
Although IS is a terrorist organization, it is also a State, holding land (about 60% of Syria and ca. 40% of Iraq) which in fact constitutes the Sunni heartland in the northern Middle East, including the swathe of eastern Syria and the Anbar province of Iraq. The population there may hate the oppression of IS, but they would certainly prefer it to the Iraqi Army, that is predominantly Shia, and the Shia militias that are Iranian dominated, which are known to carry out massacres of Sunnis. Also they would prefer IS to once again being under the thumb of the Alawite-dominated regime of Pres, Assad, which is also an anti-Sunni sect allied to the Shia. So destroying IS in this region will not be as easy as it might seem.
Certainly IS have a relatively primitive army, but they have the possibility of large numbers of Sunni recruits who are prepared to fight and die against the Shia-Alawite alliance. IS has also developed the infrastructure of a fledgling state, including an oil industry and the establishment of a welfare system. The oil income and population support measures that IS have implemented will mean that a significant percent of the population would prefer to live under IS than take their chance with any Western-backed Syrian coalition that might not support the Sunni population. This will make defeating IS a much harder proposition for the West. As far as Israel is concerned the best position is to stand on the sidelines cheering them all on.