I think it's pertinent that OP implied this was a one off, as when someone queried how she was this worked up over a one off it must be more than that, OP put them in their place.
So when you take the situation as per OP. Which is that DH typically finishes WFH by 5.30. And yesterday at twenty past 6, he's still working, 6ft away from her in the dining room. She asks if she's BU to be "seething" with him, why can't he have the decency to say he's working late, or come and help her.
It was pointed out to her yes she is BU because she could simply ask the person right in front of her. Or that he perhaps didn't think it necessary to say "I'm still working" when she's right in front of him and can see that. And quite relevant is that it's only 50 mins extra than usual when she starts her thread. Basically, she's collected DC from nursery, and on return, DH hasn't put his laptop away, which is a one off. And she's seething that he hasn't said anything, despite her being able to see what he's doing, and claims she can't ask him "how much longer will you be" (which is the obvious thing to do to someone right in front of you) because she's irate.
He stops working just before 7.
The AIBU is essentially: AIBU to be seething that DH has worked 50mins (to become 90mins) later than usual, whilst sat in front of me, without declaring this, and I've had to do the bedtime routine on my own, this once. And the consensus is yes, if you can't speak to your own husband about how long he will be, because you've got into such a grump over this one off occurrence, yes, yabu.
What OP then descended into, I'm not entirely sure, but echo PP that I hope she's ok, as the direction she took was quite extreme, and I'm glad people called her out on it, because no poster remotely called her a worthless piece of shit, and I hope she takes the advice to speak to someone.