Being racist, sexist, homophobic, abusive or a self-absorbed condescending jackass, being selfish and disloyal to their partner, being narcissistic (very different than having self-confidence which makes people attractive), not being true to themselves, etc. everyone can be attractive to the right person, but everyone can flip that switch with one shitty act and make themselves immediately look like a troll.
I know a girl who I thought was super cute and nice. Soon after meeting her, I realized she can’t let people finish talking. She tries to finish everyone’s sentences (and never gets it right) so she can start talking again. You can tell she never gives a shit about what anyone else is saying.