Complex sentence: The next day they recieve threating letters knowing what they did and that they will be punished they watch the news and discover that it really doesn't even feel like christmas as if someone killed Santa Claus.
Split into simpler sentences: The next day they recieve threatening letters stating that someone knows what they did and that they will be punished. They watch the news and discover that no one really feels like it's Christmas, as if someone killed Santa Claus.