Didnt the first DLC take place a few years after the ending of the disc? Im pretty sure thats what it said in the first level. Personality traits can change by then...especially after all the crap Gabriel went through.
No, it started right after Gabriel defeated Satan. Pay attention to the narrator. He says that after his victory felt "hollow" and that he had nothing left and as he stood there contemplating the abyss, he heard a voice in his mind pleading for help. That voice was Laura.
What made him so dark so fast was the fact that he had nothing left to live for. He felt betrayed by the brotherhood (or at least it didn't mean anything to him anymore) and his wife (who he apparently loved more than life itself) was gone for good. All that considered, I wouldn't care about anything either. Becoming a vampire was what finally turned him evil. Absorbing the Forgotten One's power probably played a part too.