Continuity mistake: When Danny dives into the ocean he's wearing different shorts than he was on the beach.
Another Killing in Cork - S11-E20
Continuity mistake: In the opening exterior shot of the train, it has at least six brown and green carriages. However, in the next exterior shot, the train is made up of three brown and cream carriages.
Answer: In a flashback scene, it was shown that he had a violent temper, so it stands to reason that he finally killed her in a fit of rage.