Faction and seriousness of the crime are all factors as to how the guards react. That guy on the bridge is a bandit of some sorts I assume since he attacks on sight.
You ran to the guards, which is a decent thing to do, but since the guards at that Fort barely know you they will take a stand-offish approach. The problem being is most likely you turned, once inside the fort, and attacked your attacker. Only the guards didn't "know" you were being attacked, which made it look like you are the aggressor and are trying to murder this guy.
Murder will make a guard run from the corners of the earth to snatch you. I've seen the guards aggro on me while I was OUTSIDE that fort cause I had a murder bounty on my head.
Yet in other instances...I've had a 40 gold pickpocket bounty and I had to speak to the guard first in order to turn myself in. They didn't actively come after me. My disposition with those guards at that time was 67/100 also. They fined me 37 gp instead of the full 40 for turning myself in.
So in essence, when you run to guards for protection, let the attacker hit you first, in plain sight of the guards before you retaliate if you can.
PS. on a side note, once you DO get a guard to either help or assist you in any way... be very carefull where you attack and what you hit. I had a cliff racer attack me at the edges of town once. I cast Soul Trap on it, then proceded to attack. In that time though, a guard had wandered close and tried to "protect" me. I swung my sword and accidentally hit the guard instead of the cliff racer. He arrested me and fined me right in the middle of battle before finishing off the cliff racer. (yes that was a "reload" situation there)
__________________
<B>Valthar</B>
|