Serial killers are often portrayed on TV and in movies as evil geniuses, but few of them are.
They are hard to catch because they generally prey on people who just aren't missed, and because there is generally little to connect them to any of their victims.
It’s just a figure of speech, grasping at a description.
They most certainly are devious, or else they would not get far. It’s not just choice of victim, though that certainly helps (that requires sone thinking). They tend to hide everything, from abducting someone to burying them, etc.
That is one of the requirements for “serial killing” - being devious and not making the killing public at all.
The DC Snipers are a curious case, not quite fitting either what should be mass killing or serial, but some combo. Never mind the fact there were more than 1.