Okay, having now played the new scene, I can see how you could possibly interpret it poorly, but I think you have to be looking at it through a very critical lens to begin with. The scene fades to black very quickly and there is no real description of how far it goes. Darius turning his back and pretending to be about to walk away is very in keeping with his character and there is no indication that he is actually planning on leaving the MC like that. Is it best practice for safe bondage play? No, not really. But does it further reinforce Darius' character and fit well with what we already know of him? It definitely does.
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The fact that Darius didn't acquiesce on the first 'let me out' is a large part of what has me concerned. I can think of three possible reasons in that context as to why Darius would be grinning alongside his boyfriend's quite obvious anger, and if I know Darius as well as I think I do, what I would consider least forgivable of the three... that Darius was deliberately provoking MC's anger for his own amusement... is also probably the most likely.
I agree with you on one point, there's no indication that Darius was planning on leaving MC there... and it could be you're right about me, too, now that I think on it; maybe I am being over-critical... but there's also no indication of how far Darius was planning to go with this before releasing MC, and even if the scene hadn't ended on that note, I ain't the sort as can trust someone who's willing to piss off their SO for a laugh; there's always part of me that wonders how much worse they might get in the future.