Point-of-View is one of those funny things writers get very worked up about. And I've noticed, reading through some older* books I have, making concrete POV choices is a relatively recent development. I mean, yes, certainly, the distinction between first-person and third-person was always clear. But third-person was often more of a muddled third-person-omniscient instead of the discrete multi-person third-person-limited, where individual scenes have a clear POV character. Even the idea of a "POV Violation" as a writing mistake seems to be a relatively new thing.
Because, let me tell you, a lot of classics are just loaded with POV Violations.
However, the standard today is for clear, discrete definition of whose head your in for any given scene or chapter. George R. R. Martin's Song of Fire and Ice books do this explicitly, telling you who the POV character is instead of a chapter title.
There are a lot of "rules" of how to do a POV character, who can be one in your book and when you can let them be one. I'm of the opinion that who can be one and when is whoever you need it to be for the scene, whenever you need that scene to be.
My big thing with POV is trust. Unless the Unreliable Narrator is a technique you're utilizing, then you have to present your POV character in an honest way. You have to trust that character and what his engagement in the plot is.
Now, that doesn't mean the POV is limited to the "good guys". I love my antagonist POVs, as long as they are antagonists that I can trust are being honest with how they engage in the plot. If I have a character who is against the hero privately, but acts as his friend, and I don't want the reader to know that... then that character can't be a POV character. But if I want that betrayal clear, then that's exactly who I want as POV.
This was especially hard for me in Maradaine Constabulary, which is probably my most constrained work, POV-wise, in that I only have Satrine and Minox as POV characters. This is because, at its core, it's a murder mystery, and if you go into the head of murderer, then the mystery is given up. By limiting the POV to my two Inspectors, then the reader has the same set of data that they do.
On my current work-in-progress, Way of the Shield, it's more complicated than that, but similar rules of not using a character for POV apply. There are people whose motivation and trustworthiness I want the reader to keep in question, even in a subconscious way. Ideally, when their truths come to light, it will hit the reader like a hammer, because they might not have even suspected it.
We'll see if I pull it off.
*- Of course, when I say "older", I'm mostly talking about from the 80s. But, of course, older than that as well.