Well, that's what perk created for ;p
To assassinate unexpected victim.
That's really bad for me to suggest this, but: May be if guards will see a sneaker in guarded town, he should break sneaker's sneak instantly. So it still will be possible to assassinate someone in dark corners of guarded town where no guards, but if you'll run to NCR bazaar, for example, and get into guards' FoV, your sneak will end instantly, then poor bluesuiters can idle in NCR more safely and don't make threads about suicidal killers 24/7.