Sure, if Assad uses chemical weapons, he will be not the only guilty, because he would been guided till this situation by the West's posture.
Syria is the last defense-line of Iran. If Assad falls, Iran can to thought it will be the next target and increases him conviction that only the bomb can make he safed. A nuclear Iran is a goal that West and Israel, and even Iran allies as Russia and India, wants to avoid.
For this, Putin defends Iran take part on Syria's transition talks, something the USA are against, saying Iran has nothing to add on talks.
So, a possible Assad's fall risks to end in a Islamist chemical Syria and a nuclear Iran, a nightmare for Israel.
Moreover, a Assad's fall can leave Hezbollah alone and weakened without Syria aid, and be a invitation for Israel attacks against the Hezbollah positions in South Lebanon, and start a new conflict round on the border zone.
And more, if Islamists rise to power in Syria, they can to pursuit religious minorities as the alawites, christian and druzes by revenge for their support to Assad or by fundamentalism against their beliefs. This would cause refugees' flees to Lebanon, which is a politically sensible country on demographics changes in their religious communities sizes.
In short, a Assad's fall can be a hazard in the powderkeg Middle East.