On the general topic of "the double and sometimes triple lives of our difficult children," I too experienced this with my difficult child nephew and his expectation that, after yet another episode of his lousy behavior (2-3 times per week, typically), all of us should simply hit "reset" and engage with him as if the last episode had never happened. In other words, he'd be a total difficult child one night (a rage, an episode of sharp conflict, him coming home drunk or high or drugged, etc) and then expect everyone to just carry on with him as if it had never happened the next day--i.e., he'd want to play chess with me, or try to engage me in "normal," cheery conversation, or talk about a movie he'd seen, or whatever. I finally "called him out" on this one day, telling him that I couldn't engage with him as if nothing had happened without a conversation about what had happened, what he had done and what he was going to do to make it right (he NEVER apologized--just expected you to "get over it" and go on as before), and that I was tired of what I called his "duality of personality"--i.e., calling me every name in the book, raging, etc, and then expecting me to interact with him "normally" (as if he were a easy child) the next day. (He had been "trained" to expect this by his mother's never-ending habit of just, figuratively speaking, "hitting re-set" and giving him a fresh start after every negative behavior/episode.) I told him that I could no longer tell which was the "real" Stan (not his real name, but I'll use it from now on, for simplicity's sake)--the hate-filled raging amoral ***hole or the "normal" easy child-like kid he expected me to regard him as the next day, or even an hour or two later, and that I suspected that the "real" Stan was the former and not the latter, and because of that, I was no longer going to engage with him when he feigned "niceness" or expected normal relations with me until he started actually addressing his behavior (i.e., acknowledging how awful it was and apologizing for it) before expecting a return to normal interaction with him.
His reply? A smug declaration that "People are complicated" and refusal to address or apologize for his bad behaviors. After that, I refused to play chess with him (previously a major pastime between us) or interact with him jovially or convivially, as I did with his younger brother--I simply couldn't do it, and told him so, without an apology from him, and an acknowledgment of wrongdoing and a promise to do better, just as I would expect from anyone else before being able to resume normal friendly relations.
What do the rest of you do about this? How do you handle "returning to normal" with a difficult child whose lousy behaviors just keep happening and for which he/she never apologizes? Speaking for myself, this was a deal-breaker in terms of interacting in a friendly way with him. After that conversation, I pretty much ignored him, almost never spoke to him, except when I had to deal with his vile misbehaviors, rages, etc.