Here are some possible explanations for the captain's behavior:

1. Incompetence - Bad decision from information available
2. Negligence - Hoped everything was OK rather than acting on information
3. Bad information from crew
4. Paralyzed by fear
5. Flawed character for leadership (see above)
6. Judgement clouded by alcohol
7. Distracted by a pretty woman
8. Arrogance - Thought he had situation under control
9. He didn't do anything wrong. We are misinterpreting available information/speculation/etc
10. Costa management/ownership was aware of this behavior (show boating with close runs to islands) from other captains and allowed it to continue.

Any others? Any votes?

My speculation is combination of the above except 3 & 9.