They don't like animals.

Folks who opt not to clean up after their dogs. Particularly in public areas.

Extreme racism.

When their opinions are... very extreme.

Earlier today, someone told me that they'd wish someone were no longer living if they mentioned liking a certain type of pizza. It definitely wasn't said in a joking tone either (not that I'd really want to be around someone who made that type of "joke" in the first place) - that was a pretty obvious indicator that they were a bad person in my book.

It was done under the guise of "I don't like to sugarcoat things" 🙄

An unwillingness to admit fault, and/or learn from one's mistakes. It's the biggest and reddest of red flags.

How they treat their waitress at a restaurant. Big red flag.