OK guys i plan to be at NCR side and after talk with the legion Vulpes at nipton he started to walk away....
Is there any reason that i sould not kill him and his companions at nipton?

unless you are deeply opposed to wearing cool wolf helmets, then nope. have fun and hopefully you picked up the incinerator in primm.

I am on my first play through. The character i created is a female goody goody, if you prey upon the weak, the helpless, or exploit them in any way i have a bullet i made with your name on it. Once i arrived at Nipton and saw what the Legion scum did to the town ALL Legion scum must and will die. As the general was leaving town i killed him, his buddies and his little dogs too leaving the scum to rot where they lay as an abject lesson to the Legion. If your Legion I am comming for you, death has a name and it is Allison.

Asai

My character didn't even talk to Vulpes. He saw what the Legion had done to the town from a hidden position on a rooftop and killed them all with dynamite and rifle fire. I got a message that I failed some quest when the Legion was wiped out. Too bad. The Kid doesn't hold with torture.