An assassin couple seeks retirement but their victims thirst for revenge, to find peace they have to kill one more time.

So… if their victims “thirst for revenge”, then the assassin couple don’t seem to be very good at their job.? Because if they were, their victims would be dead, wouldn’t they?

Well, I presume you mean their kin or comrades want revenge.? Whatever, the bigger potential issue I see?is:? why should we want to emphasize or at least be interested in the fate of a couple of assassins –professional ?murderers?? Why should we root for them to survive? ?Maybe the killing couple have ?it coming to them — it’s payback time.

Unless the couple only assassinated people who had it coming to them — oppressive dictators, corrupt businessmen, etc.?– but there’s nothing in the logline to indicate that.

Also, a specific antagonist needs to be identified among the many who “thirst for revenge”.