Roots of a Thorn Bush lyrics

"It's the quiet ones you gotta watch, you know that one eh?
Every time you see a story about a serial killer on TV, what do they do? They bring on the neighbor, and the neighbor says "Well, he was always very quiet." And someone in the room says--
It's the quiet ones you gotta watch
This sounds to me like a very dangerous assumption. I will bet you anything that when you're watching a quiet one, a noisy one will f*cking kill you. ... Who you gonna watch?