I think we all want to understand why he did this and how. That said, I know some honest-to-goodness true narcissists that are former patients of mines, one with a dual diagnosis and none of them behave like Steven. Not even close. Steven seems to be in a state of self preservation. He effed up, regretted it and had to play the role of "I didn't do it". Why did he kill Charli? Because he's an idiot. He valued his life over that of someone elses. And while this is no where near the same on a grand scale, remember the times you've run a red light. At that moment you were valuing your life over that of someone elses. Justify it as you may, you broke the law and you didn't feel like braking.
No personality disorder is needed to lose all sense of reality and kill someone. He is capable of love and affection, he just didn't give any to Charli. I hate to use this as a copout for his stupidity but the boy liked video games. While playing video games you maim and destroy with one goal; self preservation.
I stayed up late last night watching Loo's testimony again, specifically the video of Charli's remains presented to the jury. Steven had his mouth covered for a good portion and his jaw was clenched. He looked horrified and grimaced. No way that was acting. I saw him draw in a big breath and squeeze his eyes with his hand as if he were reliving the moment and wanted to erase it from his mind. I don't think he'd ever seen his crime in the daylight. He was particularly disturbed by the clumps of red hair. Why?
It's time for him to confess and let her loved ones grieve for her. The testimony is only going to get worst.
To piggyback on this I think it's possible he did the heinous above and beyond because he believed it was more heinous than what he actually would do or was in his heart and that those who knew him would think so too and so not believe it was him and think it was some kind of serial killer instead. Maybe insane coke-filled night for that one.