Ranger rep is different than NCR rep.
It's -25 per hit in random encounters.The reason you didn't lose reputation is because there was only one ranger left and that ranger didn't have a radio on. Most of the rangers do have radios on them, though, if you ever encounter a bigger group.
That's really how it works? So if there's 3 rangers, and you steal all of their radios, can you kill them without losing rep?