Was Galilee not part of Israel (the Northern Kingdom) after being destroyed by the Assyrians? If so, why were Galileans not stigmatized to the same degree as Samaritans?

The region of Galilee is in Northern Palestine, and thus would have formed part of the Kingdom of Israel. The Israelite population of Galilee was deported by the Assyrians (cf. 2 Kings 15:29). This region would have been repopulated by Gentiles from other parts of the Assyrian empire (like the Samaritans). Under the Maccabees this region was repopulated by Jews from the south (i.e. Judaea). The Jews of Galilee were ethnically Jewish and kept the Temple customs. The Samaritans on the other hand were ethnically Gentile and kept their own customs.