You can do a hidden sneak-crit headshot with a silenced weapon to kill an NCR Ranger and steal the badass armor without losing favor with the NCR... think you lose a bit of karma though. Just save before you try it, and if it doesnt work out, re-load.
You only lose Karma and gain infamy if somebody sees you or something else in special cases.
Don't plan on being able to kill any of them even with sneak criticals if you're playing on Very Hard difficulty. Even if you're right behind them and in VATS with a 95% head accuracy.
It's probably possible with psycho and a number of criticals if you're lucky.