Even if there is no magical or plot twisty explanation given for Killian’s abrupt turnaround and switch over to Dark!Hook (which I’m POSITIVE there is) I don’t even need one.
Hook is a very persona and identity driven person.
He is a person who lives in extremes.
He is a person who dons costumes and completely assumes a role quickly and easily to cope with the world around him and handle his own emotions.
He accepts the reality he is given until something challenges that reality and then he adjusts and changes accordingly.
These different faces and facets of who he really is are his version of Emma’s walls. They are the armor he wears to protect his true, more vulnerable self, from the world. They are the buffers he has in place to keep from having to deal with himself, who he doesn’t appear to like very much.
It is not until he begins to open himself up to Emma that he starts the process to blend together these different extremes, but even then he is still playing a role.
Killian’s easy adaptability to a change in state is a stark contrast from characters like Rumple and Regina who appear to struggle with each new role they are given. It takes time for them to accept the change, and they require a lot of external reassurance to guide them through it.
Regina embracing the darkness was something Rumple worked at for awhile. She didn’t succumb completely until she had a final push after she was unable to resurrect Daniel. Her switch back to the light has been no less back and forth over the seasons and even as late as 5.02 she still required a lot of external reassurance to accept her role in helping to protect the town.
Killian however has never really sought assurance from others, he is uncomfortable with it and even when he receives it, he rarely listens or accepts it.
His easily changeable nature is also fairly different from characters like Emma and her parents who assume a role and tend to stick to that role unless a large external force determines otherwise. Any changes in identity for them are brief and fleeting and afterwards they, for the most part, return to the same people they used to be.
Emma maintained her identity well after she took on the darkness, with some struggle, and only after an extreme event did she give in completely. Even now, fully succumbed she seems to be more Emma than The Dark One most of the time.
Killian, however, does not operate this way.
It’s completely in character for Killian to be told “You’re the Dark One now” and for him to immediately throw all of himself into that role, especially considering that his anchor, the person that has essentially tethered him to this ambiguous gray space he’s dwelt in since their Neverland journey, is the same person who just tethered him to that darkness.