Dunno why, but people for years have washed the image of Tieflings to the point that if you hate them they believe you are a bad person even tho they are literal demonic creatures that in past editions were evil.

Plus, as a Paladin, specially if you are lawful good, that shouldn't have broken the oath. You were doing the objectively good thing, but the DM is clearly invested in the story and is showing favoritism. You should drop that fag