China and Russia dislike any kind of interventionism because they believe it is a card for America and the west to play and incite rebellions/lead to invasion.
While some people can say China is afraid of its own people and west's "White Knight" intervention could be used to free regions like Tibet, Xinjiang and similar areas, other can cynically argue these interventions in the past are not to the benefits of locals
but seemingly help the profit of western companies.
Plus, the west don't exactly have that great of a record in "regime change" anyway.
Does this mean it is ok for dictators to nerve gas/murder/arrest its own people? No, probably not. But adding western weapons and troops into the mix is no guarantee for success.
As for Syria, I think I will sit back and watch-If the rebels are meant to win, they will. If Assad proven to be stronger, o well. If it turn into a Somalia at the throat of Israel, with Jihadis running amok and no government for anyone to hold responsible....