He has shown you who he is...... believe him. He would either fight with you or avoid you, rather than deal with his alcoholism. You were only 4 months in, I know it's heartbreaking but be grateful you didn't spend more time on the same roller coaster. Be proud of yourself for recognizing this wasn't okay with you and setting a boundary early on. You saved yourself future heartbreak.
You will never understand it, and will drive yourself crazy trying. The non-alcoholic can not get the alcoholic's stinkin' thinking. What matters is you are free to find a healthy partner that you can build a life with.