So many to choose from really (with hindsight and admitting it to myself now)
But the real defining moment for me was when one of our cats was injured.
Bit of a long boring story but at the time the cat officially belonged to the neighbours who were mistreating her and were being thoroughly unpleasant to me whenever I tried to speak to them.
I'd been looking out for the cat, giving her a bit of food, shelter etc.
Anyway, one day she came in with blood on one of her front paws and evidently in a lot of pain.
I was frantically trying to think of what best to do - she needed a vet asap but if I took her posed as her owner said I was her owner was scanned for a chip I'd be exposed as a fraud, welfare of the animal on the other hand etc etc. Owners didn't want to know.
I was tying myself in absolute knots trying to make a decision, all the while ex was sitting in the living room eating and playing video games, fully aware of the distress I was in.
All requests to help me make a decision or come with me to speak to the owners again were met with 'there's nothing you can do, leave it alone' and 'I don't know, nothing to do with me is it'
Eventually I took her in to a local vet only to discover that all four of her paws were bleeding and the claws virtually stumps (vet reckoned near miss on being hit by a car)
Sorry, long saga for this basic point - I couldn't believe that ex would just leave a creature to suffer because he just could not be bothered and to happily leave me flapping all by myself as well. It brought a lot of other instances into focus where he would just refuse to get involved in anything that was affecting/upsetting me as well, so bit of a light bulb moment
I actually heard my internal voice say 'this is a deal breaker for me' that day.
Even plucked up courage to say to ex - I can't believe you were willing to let the cat suffer like that for your own convenience. Cue whine, snivel, not my place to etc etc etc 
Still took me another year after that to end things (with the able assistance of MN, so glad I discovered this place)