in the movie Bittermoon 1992 directed by Roman Polanski; why does the character Oscar shoot Mimi ?

Why didn't he shoot her earlier and instead chose to shoot her on the cruise?
I don't get why the ending has to be that way.

and when she's dancing with Nigel he says "I know how your story ends" she replies "no you don't"
whats that suppose to mean?

I guess I'm not satisfied with the ending... how did things go with Nigel and his wife?

Somebody give me a satisfying explanation plz. I love this movie so much but its bothering me and I've rewatched it so many times

Oscar shoots Mimi because she is mainly the reason he is in a wheelchair and she shows no remorse after meeting up with him.

Bitter Moon - Mimi's dance