gonna see if literally translating this will help . . .

Woe to the conquorer - the victor of a fight is sad because he/she has won
An act of contrition - fought to repent for a sin he/she has committed
The only suitable music - the only suitable music
Would be a lacrimosa - as i understand it this is a sad piece of music, and the final work of mozart committed to paper

and we know it's an event . . .

is it when cid destroys home with the airship?

he "conquers" the fiends and guado inside, though it saddens him to do so. possibly the sin he has committed is kidnapping summoners to stop their pilgrimage