I believe there is no such thing as constructive criticism - all criticism is destructive. Criticism is a way of saying "I'm better than you" or "I know more than you." Has anybody ever changed for the better because of criticism?
The key to criticism is, to put it into Julie Andrews terms, to flavor the sour taste of the criticism with a little honey.
Obviously you/they liked something, or you wouldn't feel so betrayed. Start out focusing on that. "I really loved how you ..." But then you switch to the author's weak points, because we all need to know what those closest to us won't admit.
Generally, I wouldn't overwhelm an author with bad news, and don't threaten to "never read his stories again". They get that is the implied threat, you don't need to get on your high horse about it. Detail why you were first attracted to the story, then specify what spoiled it for you, and phrase it as such. "I couldn't deal with when you turned Charlot into a pig, because it seemed inconsistent with the main character's personality." There, you're not only specifying what you didn't like, but why it irked you so much.
If you're objecting simply because of political or moral positions ("You're a disgusting piece of shit because you ..."), you'll never efface a change. People wont' change their entire personalities to suit your individual taste. Instead, suggest what they can do to make the story better. ("Leave the politics out of the story" is an empty suggestion, as they wouldn't write a story if they didn't have something they wanted to say. Instead, suggest ways they could have gotten a similar message across with insulting quite so many people (then again, many authors main objective is to tweak as many people as they can).
Lastly, don't issue ultimatums. If you don't like a story, then simply don't read any more. You don't go on a vendetta, threatening violence and death for their positions. That just labels you as being unhinged. Instead, if they suggest a compromise, consider whether it's for the better or only makes things worse. Again, you can't dictate the story, since you didn't write it. Instead, view it from the point of view as whether a change makes the story better, rather than if the author caters to your every single desire.
Some of my best feedback letters are the "I love your writing, but ..." variety, where they say how much they like your writing, but something struck them so harshly they couldn't continue. I'll often seek the writers of those letters out just to ask about the details. In almost every case, their real objection wasn't what they're arguing. Typically, it's something completely unrelated having to do with how the story was constructed. By getting them to discuss it, it's fairly easy to identify what the actual issue is, and that information is vital, as it tells you which areas of the story to work on.
I had several readers say they hated one character, so I drilled down and identified what they disliked (that it was a strong, opinionated woman, which certain people hated). By acknowledging that in the story, and delving into her personality to have her explain why she was that way made a world of difference, as it made an objectionable character relatable. They could understand why she was the way she was, so it was no longer an obnoxious personality coming into the story from left field.
"I loved the story" responses don't really do anything beside stroke your ego, but they aren't informative at all. Specifying why you loved them helps, but you're not actually adding to the author's well of skills. By specifying what they did wrong, and why it bothers you provides them with an additional quill in their quiver they can use in future stories.