I’m not sure Assad’s goal was to kill Christians, per se; but the major areas in which the rebels did occupy were also Christian areas. That’s one of the reasons why the Russians are involved in Syria - to safeguard the orthodox congregations. Syria has the largest number of orthodox believers outside Russia.
From my reading of the Israeli press last week, it appears one of the major agreements from Helsinki was that Putin agreed to guarantee that all the Iranian forces that had been building up along the Syrian border with Israel would be removed and the Russians would keep them away. The arrangement that Trump discussed was done with the agreement of the Israeli government.
Alawites are not Moslems but a syncretic religion mixing up Shia'ism, Christianity and pre-Christian beliefs. The Islamic portion is purely in name