I think this is a very wrong line of thinking. Just because the killer did what he did doesn't mean he is abnormal (or at least doesn't appear to be to the public). Similar cases to this one were mentioned in the thread, but let's focus on the - 2004 Napa murders or Daniel Marsh. Both were individuals who appeared normal on the outside and people couldn't really fathom that they could do something like that. Sure, Marsh had his quirks, but friends nevertheless seemed shocked at what he did. Both were inexperienced and had no prior killings to their name. Both were stabbings (personal and rage filled). Both killers left little to no trace and in the case of Daniel Marsh he basically left virtually no evidence for the police to followt. Marsh at the time was only 15 and while his victims were very old, he was still basically just a kid. And if he wasn't for his bragging and basically confessing (with his friends and girlfriend initially took as a joke) he likely would have never been caught
I fail to see why a 20 year old student wouldn't be able to kill 3 girls. Ethan would be the only one to put up a fight given his large stature, but against a combat knife and being drunk and sleepy, nobody has a chance.
Look at the Delphi killer. People were speculating for years how brazen and experienced he must have been to just escape like that, leave little evidence and be capable of such brutality. Turns out the guy is just your average Joe with a happy family and everything. There's really absolutely nothing that would make you think this guy is a killer.
I don't know the personality of the killer or anything about his personal life, but I do know this with almost a certainty - he likely knew the house, hated the victims personally or what they represented, planned this for some time and knew the area exceptionally well. The area is specifically a huge clue because this guy went in and out without so much as being tracked apart from some ghost car we don't know if its' related or not. And I bothered researching the area - it's literally built like a maze. There's only two roads leading out of the whole area - through Taylor Ave where there are a bunch of cameras or Walenta Dr from which there are only two real "escape routes" from. The rest are dead ends. This speaks volumes as to the knowledge of the killer of the area to me. He either lives there, has lived there or knows the area very well (parties and such).
You also have to take into account that "picking the urge to kill" is highly unlikely given that he either knew when the victims would be home or he stalked the house extensively prior to them going to bed. The reason for that is that certainly this guy didn't just wake up one day and said "hey, i'm just gonna go out to this house and kill at 3:00-5:00 AM in the morning". He specifically waited for these victims, who came home late, to go to sleep. If he wanted to kill for the sake of killing, the roommates downstairs were there earlier.