Imagine your otp were childhood friends and over the years, they separated leaving A isolated. Person A finds the hit man profession and lives on that. Person C comes in and orders a hit on Person B. 

Person A doesn't recognize Person B and takes C's money to do the job. They find B and kills them (in any way you want) after they kill Person B, they realize it is Person B. 

Person A starts to cry over B's dead body asking B to wake up.