Originally Posted by
Windmills 
To me it's unacceptable to use a 'blackout' as an excuse to abuse another person.
I agree. If you are prone to 'blackouts' and have a tendency to abuse the people you 'love' when you blackout, then it's YOUR responsibility to prevent blackouts from occurring.
When they choose to take that drink, if they have been told they engage in abusive blackouts, they are choosing to abuse.
As for OP's question, I think it's a combination of things they really believe about you but have the self control not to say sober, things they believe about themselves they project onto you and just plain nasty bullying rage where they will say anything if they think it will hurt someone else. They just want to hurt others and they don't care who.