Most likely for the same reasons almost all crimes of this sort happen. He made a pass on her that wasn't well received, things got out of hand, emotions took over and at some point he either injured or killed her in the heat of the moment, then panicked and tried to cover up the situation.

Violent crime is rarely based on rational, well reasoned acts. I wouldn't try to read too much logic into it.

Do you have any data to support the claim that most crimes of passion involve "him making a pass at her"? This seems like an obvious narrative for a TV show but this is merely one of hundreds of possibilities that could result in her accidental death.

My "most" claim was more generic, in that they are situational, triggered by emotions (and lack of emotional control), and most often between people that know each other. Him making a pass at her is merely my conjecture of how that might have happened in this particular case, based on the circumstances that have been reported.

Pure conjecture - the sub was sunk in an attempt to destroy evidence; after all, exposing a crime scene to sea water probably doesn't make it simpler for a forensic investigator to figure out what happened.

The murder itself? Try to cover for another crime? Again, (obviously) just conjecture.

I don't get it. If you're seriously going to go with the story "I dropped her off ashore, something else must have happened to her" then you don't want to attract attention to yourself, so you definitely don't sink the sub.

Conversely, if you feel you need to sink the sub to destroy evidence, then you don't say you dropped her off, you say she died in the same "accident" that caused the sinking.

Criminal cover-ups are often impulsive and full of logic failures in retrospect. Like other plans made hastily under high stress, really.