Yesterday I killed a dragon with the help of a patrol of Whiterun guards, immediately after absorbing it's soul and with 4 guards standing next to me in awe, a thief ran up and demanded I give him all my valuables. It went about as well for him as you'd imagine.
The day before that, a skoomer dealer tried to push some drugs on me. When I pointed out to him that this was illegal, he called me a snitch and tried to murder me. He was unarmed, unarmored, outnumbered and died in about 2 seconds.
It's like all the criminal types you encounter in the world are suicidal. I know they only exist to give you more things to kill but would it hurt if they at least pretended to value their own lives and occasionally make good judgement calls about who is and isn't appropriate to mug? Pro-tip - the dragonslayer in daedric armor carrying a flaming greatsword isn't appropriate to mug.