I was thinking the same. I mean given the circumstances and the information about the drug it seems more like that the Duke drugged him and he was gonna keep him after that. Like black mail but since Alexa went with somebody else and the Duke lost it, then he went to find 0rther people with black hair and started killing them. Time period of serial killing is the same as of that night.
Just my thoughts. U can ignore them.

I think Rafael drug Louis, thats why he's acting like a drunk and can't remeber who's the person, and when maetel order Louis to kiss him, he just do without hesitating...