Suppose we put this in a non-diving context.
A man, an experienced climber, forces his fiance to take up mountain climbing. She gets a minimal amount of instruction in an artifical environment before hubby drags her across the globe to the Alps for her first climb, an admittedly not challenging one, but not an easy one for a novice..
As the couple, with a group of climbers, start out, he feigns an equipment problem (later proven to be false) and as he "fixes it", he and his new wife become well separated from any other climber (all of them admiring the scenery and not worrying about the two climbers lagging behind anyway).
One hour later, he goes back to the base camp and says his wife is in distress. He claims he "ran" all the way down, endangering his own life on the icy slopes in his haste (in the dive case, the man says on videotape that he "rocketed" to the surface and feared "the bends"). He claims his wife slipped and started down a steep ledge 200 meters up, and he went down after her but she slid away and fell to the bottom. He tried to pull her up, but he couldn't.
He is unaware that her body has already been found, dead, and recovered at a spot NOWHERE NEAR where he claimed it should be (the diver said she descended at a spot that would have landed her on the wreck, where, in fact, she was found well away from the wreck). The lethal fall site is considerably afield from the trail they were supposed to be on (later analysis showed that, for the woman's body to be on the ocean floor where it was, the couple had to be well away from the planned route when she started her descent).
The climber is also unaware that he is carrying a global positioning device that PROVES he a) never went after anyone falling down a slope, contradicting his claim that he went after her as she fell, and b) he actually climbed back down at a snail's pace, hardly in any rush to get help.
The climber tells investigators that he grabbed several other climbers as he descended and tried to get them to help, but they couldn't understand him. All other climbers testify that this is false (the diver says he grabbed and shook several other divers, who all say this is nonsense). Another lie.
Even if he didn't shove her off intentionally,
the act of forcing someone into a dangerous situation they are unprepared for, then rendering no assistance when they get into trouble can be lethal by itself. This is negligent homicide or at least civilly actionable wrongful death. The climber may not have pushed her, all he has to do to kill her is to take her to a ledge unsafe for her abilities and let her fear and inexperience kill her. However you look at it, its wrong.
Likewise, even if this guy didn't shut off her air, the act of dumping his novice wife into a strong current in the Great Barrier reef, taking her to 100 ft depth, well away from other divers, and then simply abandoning her to her fate might have been enough to panic and kill her.
One of the true crimes programs recently ran a story about a man who took his new bride to an Alaskan adventure (even though she hated the outdoors). The first thing they did was kayak in frigid waters (he had a wetsuit, she didn't need one, or so he told her). Mysteriously, her kayak capsized (what a surprise) and he decided to tow her to shore. She wanted to flag down a boat, he said no. Finally, she wised up and flagged down a boat herself and survived, although she did get hypothermia. This was a case of putting someone into a bad situation and hoping nature will take it's course.
Later in their "adventure", he took her rock climbing (even though she was obese and hated heights). Oddly enough, she fell to her death! A jury acquitted him of murder (after all, she "fell")...another jury, trying him for insurance fraud (he took out millions off life insurance on her prior to their "adventure") wasn't so stupid. They sent him to prison.
