This plot's Interrupt ability is triggered after the winner of the challenge is determined, but before he is "declared". Killing the only defender will not make the challenge unopposed, or let you win by 5+ STR -- these factors have already been fixed.
The interrupt is triggered before any "If you win the challenge..." delayed effects resolve (e.g. The Lord of the Crossing), and before any "After you win/lose a challenge" reactions trigger.
- If you trigger "The Rains of Castamere" and reveal The Red Wedding, it is too late to use the plot's Interrupt ability during this challenge.