Red flags I have encountered in various relationships:
'Gently' or 'joking' sexist, racist and anti-semitic comments early on - of course, he was totally and wholly sexist, racist and anti-semitic, but I'd stupidly given him the benefit of the doubt.
Any reference to a 'psycho' ex - no understanding at all that it was his shitty actions which had made the exes behave a certain way.
Calling his mother a 'fucking bitch' - run, just run.
Questioning me all the time, eg I'd say I was thirsty, he'd ask me if I was sure/ tell me I couldn't be thirsty etc - now I understand gaslighting, this is how it starts.
Never accepting any boundary, from the simple 'I want to go home now' right through to 'I don't like anal' - any boundary was questioned and debated, usually by suggesting I had a character flaw eg boring, prudish.
Arguments were not simply differences of opinion, to be resolved by trying to understand the other person and reaching a compromise, they were always due to me having a personality defect eg argumentative, demanding, controlling etc.
Having a child and abandoning it - being delighted and crowing at not having to pay maintenance.
Not introducing me to his friends - our lives were never going to become enmeshed. Plus he told so many lies, he couldn't have anyone joining up the stories.
Him not having any friends/having friends who didn't go back very far/ seemingly very lightweight friendships, and then suddenly falling out with them for vague/unspecified reasons and cutting the friend out of his life.
Physical threats, intimidating behaviour.
Large age gaps where he's the older partner. No, he's not sophisticated and you're not 'mature for your age', he's just a twat who can't cope with a woman his own age who would want to be an equal partner, and younger women are not likely to have experienced enough male bullshit to spot what's going on.
Buying you generous gifts which always suggest self-improvement is required, eg teeth bleaching voucher, clothes in a size 10 when you're a 14 etc.
Always being late, especially early on - doesn't matter whether he always has a genuine excuse, why is your time not as valuable as his - basically you're being trained to accept his poor behaviour.
Porn/escort fiend.
Looking at this list, I feel sorry for my younger self.