He always drank. He just didnt drink the night he killed the first group of girls because he wouldve been arrested for DUI. He got off because the only proof they had to go on was the drunk/high girls hit HIM. Earl McGraw had a sixth sense that Mike was a killer, but he didnt want to bother pursuing it.
I dont think he felt guilty over killing the other girls, he just didnt want to do another head on collision because he probably wouldve been arrested. Also, he probably thought Zoe was dead when he spun out. He definitely got off from what happened, thats why he was laughing and all happy before Kim shot him. He had shot his goo by that point. Thats why he was all fucked up when they started chasing him, he was spent.
