Why do Bones and Booth always asume the person who killed the victim is always scared au nevus?

heeeresjoyce is right, they don't always assume the killer is going to be nervous. And yes it is fairly likely that the killer will be nervous, especially where a single murder is involved, because a lot of those murders have specific motives and are people the murderer knows increasing the likelihood of them being caught. I would guess that it would really only be serial killers and sociopaths that aren't nervous.