What exactly happened? You're being killed even in towns where you never kill anyone at all? You just enter town, and then evil guards killing you for no reason or what?
Also may be you killed one of guards once? So now he's aggroing on you to revenge, but other guards killing you before he do, so he's not satisfied yet, until he'll do final damage. But you said you're dying in every town, what means you're in enclave/raiders, but you also said that you haven't joined any other faction, it means that you have bad rep, but you said that you don't have bad rep with anyone, then it can be only aggro guard, but I doubt you have killed at least 1 guard at every town, so it's something other what I don't know.
May be guards don't like you and it's NPC ABUUSSEE!!11
Btw sometimes guards in NCR killing for no reason too, I was witness of many things, like 3 guys were talking with each other for something like 10 minutes, then one of guard killed one of them, and there were no dynamite, no bad rep, no BoS/rangers around, etc. there were almost no other player, because of was in time where server has really low online, so guards killed one guy, then 2nd, then I thought that it'll be good idea to walk on exit grid before they kill me. I still don't know what happened, also I can remember 1 session ago guards killed me for no reason too, no one planted dynamite into me, I haven't killed any guards, but Sha Enin was angry on me, but he was very far away, so I was standing and standing, and then some guard decided to kill me... shit happens.