If you are a new reader, you might want to subscribe to “Holiday Wars” as well as start reading it from the beginning. Once you’re caught up on the main storyline then checkout the side stories in “Tales of Holiday’s Past.”
Remember, Fenris (the wolf) is prophesied to kill Odin. So by freeing Loki (and by association Fenris) Sinterklass has literally brought upon Odin’s death. Seems like a good enough reason to be angry.