As for the attacker at the Berlin Christmas market (not a festival), we don't know his/her religion. The only individual arrested was released for a total lack of evidence, and even then his religion was not mentioned in any news report I've heard. The Islamic State claims they inspired the attacker, but without a suspect we can't confirm that. It could have been a Muslim; it just as easily could have been an athiest waging the "War on Christmas", or a random crazy.
As for the Turkish police officer who killed the Russian ambassador, after the shooting he only shouted about remembering the Syrians killed by Russians in Aleppo, which would tend to indicate this was a purely political act. None of the reports have mentioned his religious affiliation and while Turkey is a majority Muslim nation, there is a sizeable Christian minority and both the military and police forces are strongly secular.
Does Islamic terrorism exist? certainly, but then so does Christian terrorism, Jewish terrorism, Buddhist terrorism, and secular terrorism. In fact, the most recent terrorism arrests in the US were a group of Christians plotting to blow up Muslims at their house of worship outside Kansas City.