What makes you instantly dislike a character?
This is an interesting question for two reasons.
Firstly, you would assume that what makes us dislike a character would be similar to what makes us dislike a real-life person, and to a certain extent that is true. This means the question can feel repetitive or pedantic on the surface: manipulative, cruel, or dismissive behavior can all immediately have us cringing away from a character.
However, there are plenty of characters we love to hate. Many more who have large or deep enough flaws that while we wouldn’t want to be around them in our real-life, we find them fascinating to watch or read about.
Secondly, characters have arcs, or at least they should have arcs. This means that they aren’t stagnant and will change, whether for the better or worse. You might fall in love with a character in the beginning of their arc only for them to take action after action that leads you to disliking them immensely. The opposite is also possible.
We, as readers, come with particular biases that will supersede the arcs of the characters and/or the desire to love a character beyond their unlikable traits. These biases are different in all of us, though there are some things that large groups of audience will lean toward.
My own personal biases mean that the following traits of a character will instantly make me dislike them, regardless of their arcs, even a redemption one, or their other attributes that might make them likable as villains.
Relationship cheating (don’t care if they apologize; don’t care if “oh, I’m gay and raised in a hard situation so married a woman to hide things and then continued affairs behind her back because culture is horrible, amiright?”; this behavior is ruinous to relationships and to people–caveat: does not include open relationships, polys, non-monogamous, etc. because that isn’t cheating)
Murdering babies (I think this one might be self-explanatory, but maybe not. Babies are innocent creatures without the ability to defend or care for themselves and killing them doesn’t show badassery, rather, it shows lack of empathy in all ways; can substitute innocent animals, toddlers, young children, etc. in place of babies)
Never taking responsibility (this one is slightly harder to pinpoint and usually is an accumulation of actions rather than one, but once the pattern has been discovered, it’s impossible for me to like the person even if they attempt to redeem themselves; you can also call this “playing the victim” or “the consummate whiner” or “entitled shithead who thinks the world owes them something.” All pretty much encompass the same idea)
These are the things I can think of that there is no coming back from. At all.