The problem with dual-carriageways is that drivers don't really pay attention once they've driven the same section a few times. Which likely explains why some people saw a brightly lit bicycle and others didn't. And it's not usual, it's how our brains work, we'll go into autopilot in such situations until something unusual invokes consciousness. Anyone that claims to be paying full attention is lying.
Admittedly, I can't imagine I'd hit 'a deer' and not stop.
I'd also agree with the entire 'careless' thing – there's nothing else we do where momentary 'carelessness' can be fatal to someone else. If it wasn't happening in a motor vehicle, where the rules are evidently different, I'm sure we'd be concerned. But ultimately, regardless of the definition, it's crass and offensive to put someone's death at the behest of mere carelessness. Sorry, your husband is dead. There's a bit of an oops on his way home. It seems to be legal terminology yet again aimed at the mitigation for drivers, who never like to face the fact that they might have done something significant wrong or that they should (and would in any other context) carry a higher duty of care towards others.