SPECULATION.
Depends on the serial killer and it's really hard to generalize.
Ed Kemper hung around with police quite a bit, while living his normal life. Keith Jesperson just went right on with his normal life, driving his truck, spending time with his family. Joe DeAngelo found excuses to travel but was otherwise apparently a normal family man (co-workers said he wasn't exceptionally talkative but friendly and organized fishing trips/invited acquaintances or friends along). Dennis Rader kept up his appearances at church, Boy Scouts and otherwise, while his family apparently had no clues about his crimes.
In short, I think many serial killers just tend to go on with their normal lives, using their mask of normalcy as their main way of avoiding detection.
People who "snap" usually snap more than once. I can't say whether this crime involved "snapping" or not.
It's possible that this person does lead a "marginal" life and LE would know some of the locals who fit that description. However, if the serial killer is a drifter, surely they would be long gone from Moscow, ID.
To me, this crime seems well-organized and very specific to the conditions of that neighborhood - and that night. Whoever it is, they spent time planning and watching (more than one evening, IMO). However, I'm open to nearly every theory on this crime, as we have so few details.
We call people "serial killers" because they kill on more than one occasion, over time. So far this crime is a mass murder. Dennis Rader started his serial killing with a mass murder and I'm guessing there are others who did so, but usually a serial killer goes through the rush of the kill, has a "refractory" period where they watch and wait (whether they're terribly worried seems to be in doubt - certainly Jesperson's autobiography shows a man who was able to shrug off what he was doing).
Then the urge to kill becomes uncontrollable once again OR the opportunity for an "easy target" becomes available. I need to read more about killers like Richard Ramirez, who may have been living in stolen cars for part of the time he was in Los Angeles.