Encouraging you to do something that just isn't right
It may not seem like much to some, but I think that a huge red flag in a relationship is when someone encourages you to do something that you know is not right.
Now, I'm not necessarily talking about anything illegal or immoral per se, but it's actually something as basic as seeking out information and confidences.
When you betray one person for another, even when it is something that seems small like relaying something they've told you in confidence, you are betraying yourself.
I know this for a fact. It happened to me. Ultimately, as time passes, the relationship will also pass. Asking someone to break a confidence is a red flag of other, more serious failings.
So, if you are encouraged to go against what you know is right, heed the red flag and get out now.