Syria has been supporting and sheltering PKK since the 80s. They literally have had training camps there. The Assad family did this to have a threatening chip against Turkey.
Turkey supporting rebels is a completely different scenario. (At least until that point) the opposition had no terroristic track record. They were just the majority of the country seeking justice from a dictatorial family from a minority ruling the country for decades. Turkey did nothing wrong against Syria, only the Assad family. When the country is ruled the way the Assad family did, the people are going to rise against the government one way or another, weather Turkey helps them or not.
Last but not least it is not backstabbing if you warn them many times about not massacring their own citizens for protesting. Backstabbing is when you officially deny it, but actually pretty boldly support an armed organization that is widely known as terrorists against your neighbors just so you can pressure them in negotiations.
As I said Turkey did nothing malicious in Syria, but even if it had done so, it would have been 100% justified as payback.