Complex sentence: However, much to Jon's disappointment and anger, he has been assigned as a steward to the Lord Commander instead of a ranger, no thanks to Ser Alliser Thorne, who did it as revenge for Jon defying him.
Split into simpler sentences: However, much to Jon's disappointment and anger, he has been assigned as a steward to the Lord Commander instead of a ranger. Jon suspects this was due to Ser Alliser Thorne, as revenge for Jon defying him.