I think people want to find a 'reason' (drugs, etc.) why CW murdered because they can only see things from their mindset. But CW has a different type of brain and he showed us exactly what he likes- and he enjoyed murdering. He sent the photo of a life sized girl doll wrapped in trash bags to SW before the murder. There was no reason to do that other than he wanted to savor what he was about to do. He lives to trick people and he liked being the one in control of a horrible secret, maybe he laughed at SW trying to figure out the photo of the way he would soon be killing their daughters and putting them in trash bags. He chose a lengthy, deliberate and personal way of killing -he could see and feel the life drain from them and had opportunities to change his mind but he finished. And, of course, consider his extreme display of 'duper's delight' on the front porch. He was enjoying himself. Maybe killing is something he has always wanted to do and he pushed it down until recently.
If drugs made him kill, like some on here are suggesting, once they wore off he would have experienced crushing regret, would have confessed voluntarily, would have been on his knees bawling, etc. But he wasn't - in fact he didn't display any signs of regret and he tried to blame everything on SW. I think he's always suppressed a desire to murder and that's why he overate, didn't let people close to him, etc., but then in 2018 his true self finally won out.