The thing is NCR Rangers are an illegal organisation from the point of view of NCR government according to Fallout 2 lore
Did we play the same game?"Technically part of the New California Republic military, the NCR Rangers are practically a force unto themselves. They are a select group that is pledged to protect the people of the Wastes much like the Texas Rangers of old and the Desert Rangers of Nevada. The Rangers are given exceptional leeway and flexibility to deal with both the intrusion of organized crime and the operation of slavers near the borders of NCR territory.To this end, they employ high end equipment (Combat Armors, various assault rifles and heavy weapons and even vehicles), numerous safehouses in the wastes and unorthodox tactics, especially against their sworn enemies, the slavers. A slaver is quite likely to end up with a slug in his head if he faces a Ranger. However, their methods don't endear people to them, and NCR Rangers typically have bounties on their head, even in NCR states."http://fallout.wikia.com/wiki/NCR_Rangers
Doesn't the last sentence state, that they are not really liked by people of NCR, even sometimes there are some Rangers, who might be free to kill for a bounty hunters?
Did we play the same game?
Slavery is illegal in NCR. The local police reinforces laws and detains slavers within the NCR, but the Rangers strike at slavery outside as an interventionist special force. That slaver building was built outside NCR's walls, and thus, was not subject to its laws. The police could do nothing about it, but the New California Rangers, being a legal, moralist branch of the NCR military committed to attack slavery inside and outside NCR, hired the Chosen to do that, as a test for his membership.When discussing the rangers we must understand that they employ violent methods for civilizational standards, but no different than the CIA of today outside of US jurisdiction. They're a metaphor to the interventionist, moralist vein present in the American government. And let's read again: the government gives the Rangers extra leeway: because the Congress doesn't want to be associated with the Ranger's slightly bad reputation in certain regions, they let them operate instead of the Army or the Police, lessening the association of the violence with the government, and achieving the government goals all the same. So the Rangers are just a hidden hand of the government pulling strings inside and outside their own jurisdiction. But being they still a legal paramilitary force within NCR, attacking some of them will incur in NCR's wrath, as much as killing a Vault City guard would in VC.
damn, you can tell us what about you talking exactly ?