I arrived in Cottowood (caesar Legion NEUTRAL), spoked to the centurion, give him so few tags. I walk to the house in the river and the legionaries was there, i sneaked behind him and kill with sneak critical.

Ok, i take the loot and back to the Cottowood, all the soldiers there Appears to not react with the murder ... Coming out of cottonwood, only one of the soldiers started to attack me and it all started too!

Is there a cheat to make them stop attacking?

Something buggy about faction kills. I was escorting a caravan into Novac and assisted in killing 3 legionnaire ambushers. Well 30 sec's after I kia'd the last guy ... the only guy I kia'd ... and that is 30 sec real time btw 3 heavy duty Legion Assassins showed up with a contract to kill me. "Caesar has ordered your death!" they cried!! WTH! They toasted me. In FO3 it took the Talon boys a while to start hunting me. That was ridiculous.