If when playing the "Ranger hates race X" schtick, would this be reasonable?

[Chimera]

I think that everyone else has pretty much answered the Question.

What alignment is the Ranger? If he's less than good, I could easily see working with that Human, with the eventual plan to turn on him when the original job is done. A 'leave no witnesses' kind of thing. Maybe even playing off the whole "if he's evil enough to turn on his own kind, he can't be trusted in the end" angle.

 

[heirodule]

There's a ranger imc whose favored enemy is aquatic humanoids. The only one we could find was the merfolk, so he decided that he would made his favored enemy thing more of a fascination than hatred. He really likes mermaids, in a sort of creepy, halfling Anton La Vey way.

Locathath and bullywugs too. (Used to include lizardfolk, but then they decided aquatic means breathes water, not lives in alot of water)

 

[TheEvil]

Lets say we have a elf ranger who's favored enemy is human, and he really hates them. Is it reasonable to work for a human, provided that the task involves killing other humans?

Well, since he has made a Role-Play decision, pretty much any Role-Play decision can be used to justify it. Maybe he particularly hates a certain group of humans. As long as that group is an overarching villian organization, should be no problem.